Metal oxide ceramic nanomaterials and methods of making and using same

ABSTRACT

Provided are metal oxide ceramic materials and intermediate materials thereof (e.g., nanozirconia gels, nanozirconia green bodies, pre-sintered ceramic bodies, zirconia dental ceramic materials, and dental articles). The nanozirconia gels are formable gels. Also provided are methods of making and using the metal oxide materials and intermediate materials. The nanozirconia gels can be made using, for example, osmotic processing. The nanozirconia gels can be used to make nanozirconia green bodies, pre-sintered ceramic bodies, zirconia dental ceramic materials, and dental article. The nanozirconia green bodies, pre-sintered ceramic bodies, zirconia dental ceramic materials, and dental articles have desirable properties (e.g., optical properties and mechanical properties).

FIELD OF THE DISCLOSURE

The disclosure generally relates to metal oxide ceramic materials. Moreparticularly the disclosure generally relates to nanozirconia ceramicmaterials and formable nanozirconia gels.

BACKGROUND OF THE DISCLOSURE

The commercially available full contour (monolithic) Yttria-stabilizedtetragonal zirconia polycrystal (Y-TZP) dental ceramics have been usedas a dental restorative material for over a decade for their superiormechanical properties in spite of being aesthetically inferior withlower translucency and lack of opalescence compared to lithiumdisilicate or leucite-based glass ceramic materials like IPS e.max orIPS Empress. While increasing the amount of doped yttria couldeffectively reduce the opacity of Y-TZP, it significantly lowers itsmechanical strength and limits its clinical use for multi-unitrestorations. The solution to this dilemma is to use nano-sized zirconiaparticles and thus to make zirconia restoration containing mean grainsize around 100 nm or less. Such nanozirconia would not only yieldhigher mechanical strength than conventional zirconia but also maximizeY-TZP's translucency by reducing the size of scattering centers.

The commercialization of nanozirconia dental restorations requires theability to produce a viable nanozirconia body of bulk size (thickness is10 mm or greater) with uniform microstructure. Such nanozirconiarestorations have not been reported so far in literature and the dentalindustry lacks the technology for bulk shape consolidation. The currentstate of art of processing zirconia is not suitable for nanozirconia forat least some of the following reasons:

1. Dry processing methods are not applicable. Due to the inherent highspecific surface area of nanoparticles, they are prone to strongagglomeration. This strong propensity to agglomerate can result inundesirable material properties when processed using typicalmanufacturing methods for conventional zirconia such as die pressing.2. Most of liquid processing methods are not applicable. For example,slip casting of nanozirconia suspension is not able to produce thickbodies due to its low suction power associated with capillary force ofmolds. Direct coagulation casting would introduce inhomogeneities andrequires high solid loading prior to casting stages. Gel casting methodsuse large amount of organics and later cause difficulty duringdebinding. Centrifugal consolidation from suspension is subject tonon-uniformities of solid loading/relative density in the bulk becauseof the segregation of nanoparticles of different sizes.

Liquid processing methods from colloidal suspension are still a usefulapproach to produce viable dental articles despite two technicalchallenges to make mass manufacturing nanozirconia possible. First, toinvent techniques of consolidating and drying bulk shapes withuniform/homogenous structure at the nano scale in each processing step;second, to invent techniques of processing bulk shape after casting,which include removing unwanted water and any processing agents duringdrying and debinding through extremely small pores/channels (pore sizeless than 10 nm) while maintaining the integrity of the nanozirconiabodies.

SUMMARY OF THE DISCLOSURE

The present disclosure provides metal oxide ceramic materials andintermediate materials thereof. The present disclosure also providesmethods of making and using the metal oxide materials and intermediatematerials. Examples of metal oxide ceramic materials and intermediatematerials include, but are not limited to, nanozirconia gels,nanozirconia green bodies, pre-sintered ceramic bodies, zirconia dentalceramic materials, and dental articles.

The metal oxide materials (e.g., zirconia dental ceramic materials, anddental articles) and other intermediate materials (e.g., nanozirconiagreen bodies, pre-sintered ceramic bodies) can be made from a gel of thepresent disclosure. The gels are formable gels, which can be formed intoa three-dimensional shape (e.g., a free-standing three-dimensionalshape) and subjected to further processes to provide metal oxidematerials (e.g., zirconia dental ceramic materials and dental articles)and other intermediate materials (e.g., nanozirconia green bodies,pre-sintered ceramic bodies) of the present disclosure having desirablefeatures (e.g., crack-free metal oxide materials and other intermediatematerials).

In an aspect, the present disclosure provides gels. The gels can beformable gels. The gels comprise nanozirconia and water. The gelscomprise a plurality of zirconia nanoparticles. The nanozirconia canhave an average size of 10 to 30 nm, including all integer nm values andranges therebetween. The nanozirconia can have various sizedistributions. The nanozirconia is uniform and homogenous (e.g.,well-dispersed) within the gel. The gel can have various nanozirconialoadings. The gel can comprise a processing agent. The gel can comprisea combination of processing agents. The gels have desirable opticalcharacteristics (e.g., translucency and/or opalescence). The gel hasdesirable physical properties. A gel is redispersible in an aqueousmedium (e.g., water).

In an aspect, the present disclosure provides zirconia green bodies. Thezirconia green bodies can be made by removal of water from a formablenanozirconia gel of the present disclosure (e.g., a shaped formablenanozirconia gel of the present disclosure). The zirconia green bodycomprises a plurality of zirconia nanoparticles. The nanozirconia canhave various size distributions. The zirconia green body is porous. Invarious examples, the zirconia green body has a surface area of 40 to 80m²/g, including all integer m²/g values and ranges therebetween. Thezirconia green body can comprise water. The zirconia green body cancomprise a processing agent. The zirconia green body can comprise acombination of processing agents. The zirconia green body has desirableoptical characteristics (e.g., translucency and/or opalescence). Thezirconia green body has desirable physical properties. The zirconiagreen body can have various shapes and/or sizes.

In an aspect, the present disclosure provides pre-sintered ceramicbodies. The pre-sintered ceramic bodies can be made from zirconia greenbodies of the present disclosure. The pre-sintered ceramic bodies areporous. The pre-sintered ceramic bodies comprise a plurality of zirconiananoparticles. The pre-sintered body has desirable opticalcharacteristics (e.g., translucency). For example, where pre-sinteredbody is translucent and has a transmittance at 560 nm wavelength of 40to 60% for a 1 to 2 mm thick sample of the pre-sintered ceramic body.The pre-sintered body can have various shapes and/or sizes. In variousexamples, the pre-sintered body has a longest dimension of 15 to 100 mm,including all integer nm values and ranges therebetween. In variousexamples, the pre-sintered body has a dimension of 10 to 30 mm,including all integer nm values and ranges therebetween, in a directionperpendicular to the longest dimension of the pre-sintered body(thickness).

In an aspect, the present disclosure provides zirconia dental ceramicmaterials. The zirconia dental ceramic can have at least 95% of allgrains by volume have a size of 10 to 300 nm and/or the average grainsize is 40 to 150 nm and/or the density of the zirconia dental ceramichas a density that is at least 99.5% of zirconium dioxide theoreticaldensity. The zirconia dental ceramic has desirable opticalcharacteristics (e.g., translucency and/or opalescence). The zirconiadental ceramic has desirable physical properties.

In an aspect, the present disclosure provides dental articles. Thedental articles can be comprised of zirconia ceramic materials of thepresent disclosure. The dental articles can be made from pre-sinteredbodies of the present disclosure. For example, the dental article is ablank (e.g., a simple shape) or smart blank (e.g., a shape closer to thefinal shape of a dental restoration). For example, the dental article isa dental restoration.

In an aspect, the present disclosure provides methods of making gels(e.g., gels of the present disclosure). The methods are based on theremoval of water from an aqueous suspension of zirconia nanoparticlesusing a semipermeable membrane (e.g., using osmotic processing ortangential flow processing). Water can be removed from the aqueoussuspension using an intrinsically induced pressure (e.g., an osmoticprocess) or externally imposed pressure (e.g., a tangential flowprocess). The water removal process (e.g., a portion of or all of theprocess) can be carried out with physical agitation (e.g., shaking orstirring) of the aqueous suspension. The water can be removed withoutuse of an exogenous heat source. The aqueous suspension can furthercomprise a processing agent. A method of making a gel can compriseattrition milling a starting aqueous suspension and, optionally,subjecting the attrition milled starting aqueous suspension tocentrifugation.

In an aspect, the present disclosure provides methods of making zirconiagreen bodies. The methods are based on the removal of water (e.g.,non-equilibrium water) from a gel (e.g., a gel that has been shaped intoa desired shape). The water can be removed by holding a gel (e.g., ashaped gel) in a controlled-humidity and controlled-temperatureenvironment (e.g., in multiple controlled-humidity andcontrolled-temperature environments having different humidity andtemperature).

In an aspect, the present disclosure provides methods of makingpre-sintered ceramic bodies. The methods are based on heating a zirconiagreen body. During the heating organic materials (e.g., processingagent(s)) are removed from a zirconia green body.

In an aspect, the present disclosure provides methods of making dentalarticles. The methods are based on shaping and heating a pre-sinteredceramic body or zirconia green body.

BRIEF DESCRIPTION OF THE FIGURES

For a fuller understanding of the nature and objects of the disclosure,reference should be made to the following detailed description taken inconjunction with the accompanying figures.

FIG. 1. Example of generalized nanozirconia processing flowchart.

FIG. 2. Exemplary nanozirconia processing flowchart.

FIG. 3. Difference between conventional zirconia (PRIOR ART) and anexample of nanozirconia.

FIG. 4. Examples of pore size distribution and green density.

FIG. 5. Schematic of an example of an osmotic processing set-up.

FIGS. 6A and 6B. DLS measurement of volume size distribution of A) anexample of a 2TODA suspension, and B) an example of a 2ESD suspensionduring attrition milling, indicating the deagglomeration of zirconiananoparticles.

FIGS. 7A and 7B. Gel viscosity vs. shear stress of A) 2TODA gel samples(Example 1, 2A and 5A), and B) 2ESD gel samples (Example 23, 8A and11A).

FIGS. 8A and 8B. Gel viscosity vs. shear rate, after yield point, of A)2TODA gel samples (Example 1, 2A and 5A), and B) 2ESD gel samples(Example 23, 8A and 11A).

FIG. 9. Top surface smoothness/roughness of examples of centrifugal-castblanks.

FIGS. 10A, 10B, and 10C. The cylindrical green body formed in Example 1A); stained and the glazed molar crown of Example 21C B); stained andthe glazed molar crown of Example 21C, picture taken from a differentangle C).

FIGS. 11A and 11B. Appearance of good (left) vs. bad (right) startingsuspensions before (0 min) and after attrition milling for 1 hour and 2hours, A) as-prepared 55 wt % suspension B) 0.5 wt % suspension dilutedfrom 55 wt %.

FIGS. 12A and 12B. Particle size distribution by intensity of good vs.bad starting suspensions, A) before and B) after attrition milling for105 minutes.

FIG. 13. Example of particle size by volume percentage versus attritionmilling time.

FIG. 14. An example of a “brown” body as described in Example 30. Thebrown color resulted from the oxidation of organics (TODA processingagent) in the green body when heated to 200° C. during the debindingprocess. The color was removed when “brown” body was heated to atemperature above 500° C.

DETAILED DESCRIPTION OF THE DISCLOSURE

Although claimed subject matter will be described in terms of certainexamples and embodiments, other examples and embodiments, includingexamples and embodiments that do not provide all of the benefits andfeatures set forth herein, are also within the scope of this disclosure.Various structural, logical, and process step changes may be madewithout departing from the scope of the disclosure.

All ranges provided herein include all values that fall within theranges to the tenth decimal place, unless indicated otherwise. Thenumbers and ranges in the specification and claims can cover valuesobtained by applying the regular rules of rounding and/or up to +/−5%.

The present disclosure provides metal oxide ceramic materials andintermediate materials thereof. The present disclosure also providesmethods of making and using the metal oxide materials and intermediatematerials. Examples of metal oxide ceramic materials and intermediatematerials include, but are not limited to, nanozirconia gels,nanozirconia green bodies, pre-sintered ceramic bodies, zirconia dentalceramic materials, and dental articles.

The metal oxide materials (e.g., zirconia dental ceramic materials, anddental articles) and other intermediate materials (e.g., nanozirconiagreen bodies, pre-sintered ceramic bodies) can be made from a gel of thepresent disclosure. The gels are formable gels, which can be formed intoa three-dimensional shape (e.g., a free-standing three-dimensionalshape) and subjected to further processes to provide metal oxidematerials (e.g., zirconia dental ceramic materials and dental articles)and other intermediate materials (e.g., nanozirconia green bodies,pre-sintered ceramic bodies) of the present disclosure having desirablefeatures (e.g., crack-free metal oxide materials and crack-freeintermediate materials, and intermediate materials having a thickness of10 mm or greater).

It was unexpectedly found that constant shaking of the suspension duringwater removal (e.g., osmotic processing) provided a homogenous gel andminimal cavitation. It was also unexpectedly found that use of a variousprocessing agents and combinations of processing agents provided metaloxide ceramic materials and intermediate materials with desirableproperties. For example, use of colloid stabilizers prevented theformation of undesirable agglomeration, which can led to crack formation(e.g., in a zirconia green body), and/or particle interactionstrengthening agents, which can enhance the attractive interaction amongparticles during drying process, can prevent cracking (e.g., in azirconia green body).

This disclosure is related to, for example, processing of commerciallyavailable nanozirconia suspensions into formable nanozirconia gels viaosmotic processing, and then to millable blanks and other dentalarticles by high throughput mass-production forming technologies likeCIP, centrifugal casting or vibra-forming. Specifically, nanozirconiaCAD/CAM blanks millable into a variety of dental articles and appliancescan be provided in green, “brown”, and “brown”/pre-sintered state.

The term “pre-sintering” (and its derivatives) means the same and can beused interchangeably with a term “soft-sintering.” Compared toconventional dental zirconia blanks which are commercially available inpre-sintered state and in the past were also available in the greenstate, millable nanozirconia blanks of this disclosure can be alsoavailable in “brown” state, a term used herein to describe conditionafter burning out organics from green body without noticeableshrinkage/densification. This is possible because green and thus brownnanozirconia bodies of this disclosure already have high densities andmillability comparable or exceeding commercial pre-sintered zirconiablanks. The methods described herein allow to produce millable blanks ina variety of shapes and sizes, for example, with thicknesses in therange of 10 to 30 mm. Nanozirconia blanks of this disclosure can be alsoprovided in a near net shape approximating final shape of a dentalarticle, i.e., as smart blanks which can be optionally sintered to fulldensity to reduce material waste and eliminate sintering step.

FIG. 1 shows generalized nanozirconia processing flow chart forcommercial scale process starting from the stabilized and concentratedsuspensions. FIG. 2 shows laboratory scale nanozirconia processing flowchart starting from the diluted suspensions. A method shown in FIG. 2also allows milling in green state (or “green milling”) and optional useof stabilized concentrated starting suspensions. FIG. 3 compares green,“brown”/pre-sintered (after organic burn-out) and fully densenanozirconia to conventional zirconia (PRIOR ART) in comparable state interms of translucency. Osmotic processing combined with forming by highthroughput mass-production technologies like CIP, centrifugal casting orvibra-forming as taught in the present disclosure resulted in greenbodies characterized by noticeably higher green densities and narrowerpore size distribution than drop cast samples (PRIOR ART described in,for example, Comparative Example 1) samples leading initially to greaterdifficulties in drying and organic burn-out as water vapor and gaseousbyproducts of decomposing organic additives should escape through anetwork of nanoscale channels, for example, with a diameter of less than10 nm—see FIG. 4. Narrower channels filled with moisture are alsoassociated with higher capillary forces making drying and organic burnout even more challenging, especially for relatively thick samples(e.g., samples having a thickness of 10 mm or greater).

It was unexpectedly found that processing improvements (such as constantagitation/shaking of membrane bag or the whole osmotic bath duringosmotic processing as shown in FIG. 5) combined with certain organicadditives (processing agents) provide uniform gels, which are capable ofsurviving drying and form crack-free green bodies and organic burn outin thicknesses of 10 to 30 mm consistent with commercial CAD/CAM blankdimensions. Hence the concept of resilient green bodies is introduced inthe present disclosure to separate from conventional nanozirconia greenbodies. It was found that to successfully burn out organics fromnanozirconia green bodies their total organic content (includingprocessing agents) should be under 3.1 wt. %.

It was also unexpectedly found that these green bodies can be milled ingreen state resulting in realistic crowns and bridges while theirprocessing agent content (e.g., particle strengthening agent) was onlyfrom 0.1 to 1.1% by weight based on total weight of the nanoparticles inthe green body (e.g., with additional processing agent(s), such as, forexample, 2% by weight based on total weight of the nanoparticles in thegreen body TODA and/or ESD) indicating completely different mechanismthan in conventional binder systems used in the current state of the artcommercial zirconia with binder/plasticizer content of 3 to 5 wt. %. Forexample, green bodies having compositions comprising only 0.15% byweight based on total weight of the nanoparticles in the green body ofprocessing agents used in conventional zirconia as binders (0.1%PEG35K+0.05% PVA9k) were successfully milled, which challengesapplicability of conventional terminology of “binder/plastisizer” toprocess and materials of this disclosure. Without intending to be boundby any particular theory, it is considered that the millability of thegreen bodies is a result of the increased density and/or increasedhardness of green bodies of the present disclosure compared toconventional zirconia green bodies.

It is noteworthy that desirable translucency is maintained through allstages of the processing from translucent suspensions to translucent gelto translucent green bodies to translucent brown or pre-sintered bodiesto translucent fully sintered bodies. It was found out that hightranslucency is important visual indicator of the success of all theintermediate steps characteristic of the present disclosure.

In an aspect, the present disclosure provides gels. The gels areformable gels. The gels comprise nanozirconia and water. The gels can bemade by a method of the present disclosure. In an example, a gel is madeby a method of the present disclosure.

The gels comprise a plurality of zirconia nanoparticles. Zirconiananoparticles are also referred to herein as nanozirconia. Thenanozirconia in a gel can have a zirconium dioxide nominal compositioncomprising at least 91 wt % of ZrO₂ or at least 99% wt % of ZrO₂ wt%+HfO₂ wt %+Y₂O₃ wt %+Al₂O₃ wt %. The nanozirconia can be doped (e.g.,yttria doped, alumina doped, or combinations thereof). For example, thezirconia nanoparticles can be yttria-stabilized YTZP zirconiananoparticles comprising from 1 to 3 mol % of yttria (Y₂O₃) (e.g., 1Y,2Y, or 3Y, where 1Y corresponds to about 1.7 to 1.8 wt % of Y₂O₃, 2Y toabout 3.3 to 3.7 wt % of Y₂O₃, and 3Y to about 5.0 to 5.5 wt % of Y₂O₃).For example, the yttria content is 2 mol % or less (e.g. 1.5 to 2 mol%). In another example, the yttria content is 2.5 mol % or greater(e.g., 2.5 to 3 mol %). For example, the zirconia nanoparticles can beyttria-stabilized YTZP zirconia nanoparticles doped with alumina(Al₂O₃), where the alumina comprises 0.05 to 0.3 weight % based on thetotal nanoparticle weight. Without intending to be bound by anyparticular theory, it is considered that yttria content can be selected(e.g., 1.5 to 2 mol %) to provide desirable strength or selected (e.g.,2.5 to 3 mol %) to provide desirable translucency of the materials(e.g., dental material) made using the gel. Suitable nanozirconia iscommercially available and can be made by methods known in the art.

Other optional oxides that may be present in gels as described herein(e.g., as optional components of the nanozirconia particles, coating(s)on the nanozirconia particles, or aqueous component of the gel) include,but are not limited to, HfO₂, CeO₂, Pr₂O₃, Nd₂O₃, Tb₂O₃, Er₂O₃, Fe₂O₃,MnO₂, Co₂O₃, Cr₂O₃, NiO, CuO, Bi₂O₃, SiO₂, and combinations thereof.Specific additives that may add desired coloring to the resultingnanozirconia ceramic or nanozirconia articles (e.g., coloring oxides)include, for example, CeO₂, Pr₂O₃, Nd₂O₃, Tb₂O₃, Er₂O₃, Fe₂O₃, MnO₂,Co₂O₃, Cr₂O₃, NiO, CuO, Bi₂O₃, and combinations thereof. In an example,the amount of coloring oxide(s) is in an amount in a range of about 10ppm to 10,000 ppm, including all integer ppm values and rangestherebetween. In another example, the amount of coloring oxide(s) is inan amount in a range of about 20 ppm to 1,000 ppm. In some embodiments,it is desirable to have sufficient oxides present such that thenanozirconia ceramic or nanozirconia articles have coloring matchingnatural dentition, shade standards such as, for example, Ivoclar'sMO1-MO4 shade standards, or shade standards such as, for example,Ivoclar Vivadent Chromascop or Vita classical Shade guides. Otheradditives such as, for example, compounds comprising rare earth elementsand/or comprising lanthanum group elements can be added to impartfluorescence and/or modify opalescence of nanozirconia. While thedopants above are described as oxides for convenience, after sinteringto full density they will be present as ions (e.g., within the zirconiacrystal lattice or in intergranular phases) and their final content inthe resulting nanozirconia article will be given on oxide basis.Initially they can be added as, for example, salts, colloids,organometallic compounds, ionic solutions, and the like, and thereforeare referred to as additives.

Nanozirconia ceramics and nanozirconia articles (e.g., shadednanozirconia) can be doped with coloring ions such as, for example, Prions, Fe ions, Cr ions, Ni ions, Co ions, Er ions, Mn ions, Tb ions, Ndions, Ti ions, Cu ions, Bi ions, or a combination thereof to matchcolors of human dentition. Base shade zirconia can comprise the baselinelevels of some or all of the oxides in the table below. Typically, lighttransmittance of shaded zirconia is 5 to 50% lower than lighttransmittance of unshaded or “naturally colored” or base-shade zirconia.Coloring ions can be added at any point during the fabrication process.For example, coloring ions can be added during any of the steps outlinedin FIGS. 1 and 2, prior to forming of the CAD/CAM blanks or shaping gel.For example coloring can be achieved by adding ionic solutionscomprising these elements before, during, or after the attrition millingstep (see, e.g., FIG. 1). It may be desirable to add coloring ions priorto semi-permeable membrane processing step. In another example,water-soluble salts are added (e.g., using a speed mixer, which canassist in homogenizing the gel and can result in the salts beingdissolved in the aqueous component of the gel) following asemi-permeable membrane processing step (see, e.g., FIG. 2).

Example of baseline level of selected other optional oxides innanozirconia:

HfO₂ ≦30000 ppm   SiO₂ ≦100 ppm  TiO₂ ≦20 ppm Fe₂O₃ ≦20 ppm Na₂O ≦50 ppmNiO ≦20 ppm Cr₂O₃ ≦20 ppm Ce₂O₃ ≦20 ppm

The nanozirconia in a gel can have an average size of 10 to 30 nm,including all integer nm values and ranges therebetween. The averagesize of the nanoparticles can be determined by methods known in the art.For example, the average size of the nanoparticles is determined bydynamic light scattering (DLS) and when DLS is used to determinenanoparticle size, the size is an equivalent size. For example, theaverage size of the nanoparticles is determined by electron microscopy(e.g., transmission electron microscopy). When electron microscopy isused to determine nanoparticle size, the term “size” can mean thelongest dimension of the nanoparticles.

The nanozirconia in a gel can have various size distributions. Forexample, 95% or more of the zirconia nanoparticles by volume (of thenanoparticles in the formable gel) have a size of 45 nm or less. Invarious examples, 96% or more, 97% or more, 98% or more, or 99% of thezirconia nanoparticles by volume have a size of 45 nm or less and/or 99%or more or 99.5% or more of the zirconia nanoparticles by volume have asize of 70 nm or less. In another example, i) 99% of nanoparticles byvolume have a size less than 60 nm±10 nm; ii) 95% of nanoparticles byvolume have a size less than 40 nm±5 nm; iii) 50% of nanoparticles byvolume have a size less than 20 nm±5 nm; and iv.) 5% of nanoparticles byvolume have a size less than 12 nm±3 nm.

The nanozirconia is uniform and homogenous (e.g., well-dispersed) in thegel. For example, 95% or greater by volume of the zirconia nanoparticlesin a gel comprise 1 to 5 crystallites. In various examples, a gel hasless than 2% or less than 1% agglomerates (e.g., agglomerates comprisinggreater than 5 crystallites) by volume based on the total volume of thegel (e.g., 1 to 2% by volume).

The gels can have various nanozirconia loadings. For example, thezirconia nanoparticles are present in the gel at 70 to 85% by weightbased on the total weight of the gel. In various examples, the zirconiananoparticles are present in the gel at 73 to 83% by weight or 75 to 81%by weight, based on the total weight of the gel. In an example, thezirconia nanoparticles are present in the gel at 28 to 48% by volumebased on the total volume of the gel. In various examples, the zirconiananoparticles are present in the gel at 31 to 44% or 33 to 41% by volumebased on the total volume of the gel.

The gels can comprise a processing agent. The gel can comprise acombination of processing agents. Various amounts of processing agent(s)can be used. In various examples, the amount of the processing agent is1.5 to 3.3% by weight, including all 0.1% by weight values and rangestherebetween, based on the total weight of the nanoparticles in the gel.In various example, the amount of the processing agent is 1.7 to 3.1% byweight or 2 to 2.6% by weight, based on the total weight of thenanoparticles in the gel. In various example, the processing agentcomprises (or is) a particle interaction strengthening agent orcombination of such agents and the amount of the particle interactionstrengthening agent(s) is 0.15 to 1.05% by weight or 0.15 to 0.55% byweight, based on the total weight of the nanoparticles in the gel.Suitable processing agents are commercially available and can be made bymethods known in the art.

Examples of suitable processing agents include, but is not limited to,of colloid stabilizers, particle interaction strengthening agents, andcombinations thereof.

Examples of suitable particle interaction strengthening agents include,but are not limited to, polymer such as polyethylene glycol (PEG),polyethylene oxide (PEO), polyvinylalcohol (PVA), methyl cellulose,polyacrylic acid, dextrin, poly-ethylene-co-propylene-glycol, andcombinations thereof. The polymers can have various molecular weights.In various examples, the polymer has a molecular weight (Mw) of 2,000 to200,000 g/mol, including all integer g/mol values and rangestherebetween. In various other examples, the polymer has a molecularweight (Mw) of 5,000 to 100,000 g/mol or 8,000 to 40,000 g/mol.

Examples of suitable colloid stabilizers include, but are not limitedto, dispersants, protective colloids, and combinations thereof. Colloidstabilizers can be steric colloid stabilizers. Colloid stabilizers canbe electrosteric colloid stabilizers and/or electrostatic colloidstabilizers. Examples of colloid stabilizers also include, but are notlimited to, organocarboxylic acids and salts thereof, polyoxocarboxylicacids and salts thereof (e.g., CH₃—[O—(CH₂CH₂)]_(x)CO₂H and saltsthereof, where x is 1 to 120, including all integer values and rangestherebetween), amino acids and salts thereof, organoamines and ammoniumsalts thereof, organoalcohols, organosilanes, and combinations thereof.In various examples, the polyoxocarboxylic acids have the followingstructure: CH₃—[O—(CH₂CH₂)]_(x)CO₂H structure or salts thereof, where xis 1 to 50 or 1 to 30.

Examples of electrosteric colloid stabilizers and/or electrostaticcolloid stabilizers, include, but are not limited to,2-[2-(2-methoxyethoxy)ethoxy]acetic acid (TODA),2-(2-methoxyethoxy)acetic acid (DOHA),O-(2-carboxyethyl)-O′-methyl-undecaethylene glycol, methoxypolyethyleneglycol propionic acid (e.g., having a molecular weight of 5,000),3,6,9-trioxaundecanedioic acid, and polyacrylic acid, bicine, dodecylamine, tetradecyl methyl amine, cetyl trimethyl ammonium bromide (CTAB),ammonium polyacrylate, polyethylene glycol dodecyl ether,trimethoxy(propyl)silane,2-[(acetoxy(polyethyleneoxy)propyl]triethoxysilane,2-[methoxy(triethyleneoxy)propyl]trimethoxysilane, and combinationsthereof. In various examples, methoxypolyethylene glycol propionic acidshave the following structure:

where n is 1 to 120, including all integer values and rangestherebetween. In various examples, n is 1 to 50 or 1 to 30.

Examples of electrostatic colloid stabilizers, include, but are notlimited to acids, such as, for example, nitric acid, HCl, and carboxylicacids (e.g., acetic acid, citric acid, and oxalic acids) and bases suchas, for example, ammonium hydroxide, tetraalkylammonium hydroxides(e.g., tetramethylammonium hydroxide), ethylenimine, urea, and salts,such as, for example, ammonium citrates (e.g., triammonium citrate anddiammonium citrate), tetraalkylammonium chlorides (e.g., methylammoniumchloride), ammonium chloride, ammonium carbonate. Electrostatic colloidstabilizers can be referred to herein by ESD.

The processing agent can comprise a steric colloidstabilizer/electrosteric colloid stabilizer (e.g., TODA) and/or anelectrostatic colloid stabilizer (e.g., an ESD). For example, theprocessing agent is 2% by weight TODA and/or ESD and 0.1% by weight PEG(e.g., PEG having a molecular weight of 35K (PEG35k)) and 0.05% byweight PVA or 0.5% percent by weight PEG (e.g., PEG having a molecularweight of 35K (PEG35k)) and 0.05% by weight PVA, where the percent byweight values are based on the total weight of the nanoparticles in thegel.

The processing agent can be covalently bound to the nanozirconia in thegel. For example, at least a portion of the processing agent is attachedvia at least one covalent bond to at least a portion of the zirconiananoparticles in the gel. For example, the processing agent is attachedto a zirconia nanoparticle by one or more nanoparticle surface Zr—O—bond.

Gels have desirable optical characteristics (e.g., translucency and/oropalescence). For example, the gel is translucent and has atransmittance at 560 nm wavelength of 60 to 80% for a 1 to 2 mm thicksample of the gel and/or the gel has an opalescence of 20 to 30 for a 1to 2 mm thick sample of the gel.

Transmittance of any of the materials of the present disclosure can bemeasured using methods known in the art. For example, total forwardtransmittance measurement was measured using integrating spheres. Tocollect all transmitted light, samples were placed up to the input portof the integrating sphere. When light strikes samples, integraldetectors collect light and calculate the total forward transmittance inthe spectrum range of visible wavelength.

The degree of opalescence (opalescence value) of any of the materials ofthe present disclosure can be quantified by methods known in the art.For example, the opalescence value is determined by a colorimetricspectrophotometry measurement with a CIE (Commission Internationaled'Eclairage) standard. For example, “Opalescence Parameter” (OP) or“Chromaticity Difference” are used as a measure of opalescence. Theopalescence parameter (OP or “Chromaticity Difference”) is calculatedaccording to the following formula:OP=[(ClEa_(T)*−ClEa_(R)*)²+(ClEb_(T)*−ClEb_(R)*)²]^(1/2), wherein(ClEa_(T)*−ClEa_(R)*) is the difference between transmission andreflectance modes in red-green coordinate a*; (ClEb_(T)*−ClEb_(R)*) isthe difference between transmission and reflectance modes in yellow-bluecolor coordinate b*. Opalescence or transmittance can be determinedusing a 1 to 2 mm thick sample.

A gel is redispersible in an aqueous medium. For example, a gel isredispersible in water. By “redispersible” it is meant that the zirconiananoparticles of the gel after resuspension in an aqueous medium have anaverage size or Dv50 that is within 2 nm or 10%, respectively, of theaverage size or Dv50 of the gel before redispersion (e.g., of thesuspension used to make the suspension).

The gels are formable gels. By “formable” it is meant that the gel canbe formed into a three-dimensional shape. The gel can be formed into afree-standing three-dimensional shape. The gels can be formed into threedimensional shapes, such as, for example, platonic solids, prisms,pyramids, spheres, cylinders, cones, discs, and any portion thereof. Thegels can also be formed into the shape (or approximate shape to accountfor shrinkage of the gel on drying and further processing) of a dentalarticle. For example, a formable gel of the present disclosure isformable into the desired shape (e.g., by centrifugal force, vibration,pressure, or a combination thereof) and capable of losing water in acontrolled-humidity and controlled-temperature environment withoutcracking while maintaining the shape integrity, whereby a zirconia greenbody having 2% to 5% water based on the weight of the zirconia greenbody is formed.

A gel can be reversibly shaped. A gel can be shaped and subsequentlyreshaped (e.g., reshaped into a different shape).

The gel has desirable physical properties. For example, the gel exhibitsa viscosity at yield point of 1×10⁹ to 12×10⁹ mPa·s. In variousexamples, the gel exhibits a viscosity at yield point of 1×10⁹ to 9×10⁹mPa·s or 2×10⁹ to 8×10⁹ mPa·s. For example, the gel exhibits a yieldstress of 1×10³ to 9×10³ Pa.

The nanozirconia particles can have a desirable phase. For example, thezirconia nanoparticles are in a tetragonal phase. In another example,10% or less of the zirconia nanoparticles are in a cubic and/or amonoclinic phase.

In an aspect, the present disclosure provides zirconia green bodies. Thezirconia green bodies can be made by removal of water from a formablenanozirconia gel of the present disclosure (e.g., a shaped gel of thepresent disclosure). The zirconia green bodies can be made by a methodof the present disclosure. In an example, a zirconia green body is madeby a method of the present disclosure.

The zirconia green body comprises a plurality of zirconia nanoparticles.Suitable zirconia nanoparticles are disclosed herein. For example, thezirconia nanoparticles can be yttria-stabilized YTZP zirconiananoparticles comprising from 1 to 3 mol % of yttria (Y₂O₃). Suitablenanozirconia is commercially available and can be made by methods knownin the art. Based on the optical characteristics of the zirconia greenbodies, it is considered that the zirconia nanoparticles in a zirconiagreen body have the same size and size distribution as the nanoparticlesof the gel used to make the zirconia green body.

The zirconia nanoparticles in a zirconia green body can have varioussize distributions. For example, 95% or more of the zirconiananoparticles by volume have a size of 45 nm or less. In variousexamples, 96% or more, 97% or more, 98% or more, or 99% of the zirconiananoparticles by volume have a size of 45 nm or less and/or 99% or moreor 99.5% or more of the zirconia nanoparticles by volume have a size of70 nm or less. In another example, i) 99% of nanoparticles by volumehave a size less than 60 nm±10 nm; ii) 95% of nanoparticles by volumehave a size less than 40 nm±5 nm; iii) 50% of nanoparticles by volumehave a size less than 20 nm±5 nm; and iv.) 5% of nanoparticles by volumehave a size less than 12 nm±3 nm.

The zirconia green body is porous. The pores can be interconnected. Invarious examples, the zirconia green body comprises pores having anaverage size (e.g., an equivalent average size) of 3 to 10 nm, includingall integer nm values therebetween. For example, at least a portion ofthe pores are interconnected. The pore size/pore size distribution(e.g., average pore size) can be measured by methods known in the art.For example, pore size/size distribution is measured by BET/BJH methodusing a Quantachrome Nova 1000 surface analyzer.

The zirconia green body can have a range of surface area. In variousexamples, the zirconia green body has a surface area of 40 to 80 m²/g,including all integer m²/g values and ranges therebetween. For example,surface area is measured by BET/BJH method using a Quantachrome Nova1000 surface analyzer.

The zirconia green body can have a range of density. In variousexamples, the zirconia green body has a density of 50 to 70%%, includingall integer % values and ranges therebetween, of the zirconium dioxidetheoretical density. In another example, the zirconia green body has adensity of 50 to 60% of the zirconium dioxide theoretical density. Forexample, the theoretical density of tetragonal zirconium dioxide is 6.10g/cm³. The zirconium theoretical density is dependent on the zirconiumdioxide composition (e.g., dopants present in the zirconium dioxide).For example, yttria-doped zirconia has a density of 6.114, 6.106, 6.101,6.094, or 6.082 g/cm³ for a yttria content of 1.7, 2.0, 2.2, 2.5, or 3.0mol %, respectively.

The nanozirconia particles can have a desirable phase. For example, thezirconia nanoparticles are in a tetragonal phase. In another example,10% or less of the zirconia nanoparticles are in a cubic and/or amonoclinic phase.

The zirconia green body can comprise water. In various examples, thezirconia green body further comprises 2 to 5% by weight, including all0.1% by weight values therebetween, based on the total weight of thezirconia green body (e.g., for a zirconia green body at equilibrium withthe ambient environment (e.g., relative humidity of 25 to 75%) at roomtemperature (e.g., 18 to 25° C.).

The zirconia green body can comprise a processing agent. The zirconiagreen body can comprise a combination of processing agents. The zirconiagreen body can have processing agents as described herein. Variousamounts of processing agent(s) can be used. In various examples, theamount of the processing agent is 1.5 to 3.3% by weight, including all0.1% by weight values and ranges therebetween, based on the total weightof the nanoparticles in the zirconia green body. In various example, theamount of the processing agent is 1.7 to 3.1% by weight or 2 to 2.6% byweight, based on the total weight of the nanoparticles in the zirconiagreen body. In various example, the processing agent comprises (or is) aparticle interaction strengthening agent or combination of such agentsand the amount of the particle interaction strengthening agent(s) is0.15 to 1.05% by weight or 0.15 to 0.55% by weight, based on the totalweight of the nanoparticles in the zirconia green body. Suitableprocessing agents are commercially available and can be made by methodsknown in the art.

The processing agent can be TODA and/or ESD. For example, the processingagent is 2% by weight TODA and/or ESD and 0.1% by weight PEG (e.g., PEGhaving a molecular weight of 35K (PEG35k)) and 0.05% by weight PVA or0.5% percent by weight PEG (e.g., PEG having a molecular weight of 35K(PEG35k)) and 0.05% by weight PVA, where the percent by weight valuesare based on the total weight of the nanoparticles in the zirconia greenbody.

The processing agent can be covalently bound to the nanozirconia in thezirconia green body. For example, at least a portion of the processingagent is attached via at least one covalent bond to at least a portionof the zirconia nanoparticles in the zirconia green body. For example,the processing agent is attached to a zirconia nanoparticle by one ormore nanoparticle surface Zr—O— bond.

The zirconia green body has desirable optical characteristics (e.g.,translucency and/or opalescence). For example, where the zirconia greenbody is translucent and has a transmittance at 560 nm wavelength of 50to 70% for a 1 to 2 mm thick sample of the zirconia green body and/orthe zirconia green body has an opalescence of 20 to 30 for a 1 to 2 mmthick sample of the zirconia green body.

The zirconia green body has desirable physical properties. In variousexamples, the zirconia green body exhibits a Vickers hardness of 35 to70 kg/mm², including all integer kg/mm² values and ranges therebetween.In various examples, the zirconia green body exhibits a Vickers hardnessof greater than 20, 25, 30, 35 or 40 kg/mm². The Vickers hardness can bemeasured by methods known in the art. For example, the Vickers hardnessis measured by methods described herein (e.g., the method described inExample 29).

The nanozirconia particles of the zirconia green body can have adesirable phase. For example, the zirconia nanoparticles arepredominantly, or all, in a tetragonal phase. In another example, 10% orless of the zirconia nanoparticles are in a cubic and/or a monoclinicphase.

The zirconia green bodies can be crack-free. For example, a zirconiagreen body is free from observable cracks. Cracks can be observed bymethods known in the art. For example, cracks can observed visually orby imaging techniques known in the art.

The zirconia green body can have various shapes and/or sizes. In variousexamples, the zirconia green body has a longest dimension of 15 to 100mm, including all integer mm values and ranges therebetween. In variousexamples, the zirconia green body has a dimension of 10 to 30 mm,including all integer mm values and ranges therebetween, in a directionperpendicular to the longest dimension of the zirconia green body(thickness).

In an aspect, the present disclosure provides pre-sintered ceramicbodies. The pre-sintered ceramic bodies can be made from zirconia greenbodies of the present disclosure. The pre-sintered ceramic bodies can bemade by a method of the present disclosure. In an example, apre-sintered ceramic body is made by a method of the present disclosure.The pre-sintered ceramic bodies are porous.

The pre-sintered ceramic bodies comprise a plurality of zirconiananoparticles. The nanoparticles can have a composition as describedherein. The zirconia nanoparticles of the pre-sintered ceramic body canhave various sizes and size distributions.

The pre-sintered body has desirable optical characteristics (e.g.,translucency). In various examples, where pre-sintered body istranslucent and has a transmittance at 560 nm wavelength of 40 to 60%,%, including all integer % values and ranges therebetween, for a 1 to 2mm thick sample of the pre-sintered ceramic body.

The pre-sintered body can have a range of density. In various examples,the pre-sintered body has a density of 50 to 70%, including all integer% values and ranges therebetween, of the zirconium dioxide theoreticaldensity. In another example, the pre-sintered body has a density of 50to 60% of the zirconium dioxide theoretical density. The theoreticaldensity of zirconium dioxide is as described herein.

The nanozirconia particles of the pre-sintered body can have a desirablephase. For example, the zirconia nanoparticles are in a tetragonalphase. In another example, 10% or less of the zirconia nanoparticles arein a cubic and/or a monoclinic phase.

The pre-sintered bodies can be crack-free. For example, a pre-sinteredbody is free from observable cracks. Cracks can be observed by methodsknown in the art. For example, cracks can observed visually or byimaging techniques known in the art.

The pre-sintered body can have various shapes and/or sizes. In variousexamples, the pre-sintered body has a longest dimension of 15 to 100 mm,including all integer mm values and ranges therebetween. In variousexamples, the pre-sintered body has a dimension of 10 to 30 mm,including all integer nm values and ranges therebetween, in a directionperpendicular to the longest dimension of the pre-sintered body(thickness).

In an aspect, the present disclosure provides zirconia dental ceramicmaterials. The zirconia dental ceramic materials can be made from gelsof the present disclosure. The zirconia dental ceramic materials can bemade by a method of the present disclosure. In an example, a zirconiadental ceramic material is made by a method of the present disclosure.

The zirconia dental ceramic can have at least 95% of all grains byvolume have a size of 10 to 300 nm, including all integer nm values andranges therebetween, and the average grain size is 40 to 150 nm,including all integer nm values and ranges therebetween, and/or thedensity of the zirconia dental ceramic has a density that is at least99.5% of zirconium dioxide theoretical density, and/or the zirconiadental ceramic is opalescent. In an example, the average grain size is80 to 120 nm.

The grain size (e.g., average grain size) can be determined by methodsknown in the art. For example, the average grain size is determined byASTM E112 (or EN 623-3). For example, the grain size is determined byimaging methods such as, for example, scanning electron microscopy.

The zirconia dental ceramic materials can be crack-free. For example, azirconia dental ceramic is free from observable cracks. Cracks can beobserved by methods known in the art. For example, cracks can observedvisually or by imaging techniques known in the art.

The zirconia dental ceramic has desirable optical characteristics (e.g.,translucency and/or opalescence). For example, where the zirconia dentalceramic is translucent and has a transmittance at 560 nm wavelength of25 to 50% for a 1 to 2 mm thick sample of the zirconia dental ceramicand/or the zirconia dental ceramic has an opalescence of 9 or greater(e.g., 9 to 15) for a 1 to 2 mm thick sample of the zirconia dentalceramic.

The zirconia dental ceramic has desirable physical properties. Forexample, the zirconia dental ceramic has an average flexural strength of1200 MPa or greater. In various examples, the zirconia dental ceramichas an average flexural strength of 1500 MPa or greater, 1800 MPa orgreater, or 2000 MPa or greater. The average flexural strength can bemeasured by methods known in the art. For example, the average flexuralstrength is measured by ISO 6872. For example, the zirconia dentalceramic has an average tensile strength of 500 MPa or greater. Theaverage tensile strength can be measured by methods known in the art.For example, the average tensile strength is measured by ASTM C1273.

In an aspect, the present disclosure provides dental articles. Thedental articles can be comprised of zirconia ceramic materials of thepresent disclosure. The dental articles can be made from pre-sinteredbodies of the present disclosure. The dental articles can be made by amethod of the present disclosure. In an example, a dental article ismade by a method of the present disclosure.

The present disclosure provides various dental articles. For example,the dental article is a blank (e.g., a simple shape) or smart blank(e.g., a shape closer to the final shape of a dental restoration). Forexample, the dental article is a dental restoration. Examples of dentalrestorations include, but are not limited to, full-contour FPDs (fixedpartial dentures), bridges, implant bridges, multi-unit frameworks,abutments, crowns, partial crowns, veneers, inlays, onlays, orthodonticretainers, space maintainers, tooth replacement appliances, splints,dentures, posts, teeth, jackets, facings, facets, implants, cylinders,and connectors.

In an aspect, the present disclosure provides methods of making gels.The methods are based on the removal of water from an aqueous suspensionof zirconia nanoparticles using a semipermeable membrane. The water canbe removed without use of an exogenous heat source.

Various aqueous suspensions of zirconia nanoparticles can be used.Examples of suitable zirconia nanoparticles are described herein. Forexample, aqueous suspensions of yttria-stabilized YTZP zirconiananoparticles comprising from 1 to 3 mol % of yttria (Y₂O₃) were used inthis disclosure yielding dental articles with a desirable combination ofmechanical and optical properties including strength, translucency andopalescence. In an example, an aqueous suspension comprises zirconiananoparticles having an average size of 10 to 30 nm and 95% or more ofthe zirconia nanoparticles by volume have a size of 45 nm or less, wherethe zirconia nanoparticles are present at less than 70% by weight basedon the total weight of the aqueous suspension. In various examples, thezirconia nanoparticles are present at 50 to 70% by weight of the aqueoussuspension or 50 to 60% by weight of the aqueous suspension.

In an example, aqueous suspensions were modified with processing agentsand attrition milled before the osmotic processing, as described indetails in Comparative Example 1 and Example 7. For example,2-[2-(2-methoxyethoxy)ethoxy]acetic acid (also known as3,6,9-trioxadecanoic acid abbreviated as TODA and TODS) and ESD, wereadded at 2% by weight based on total weight of the suspension to thezirconia solid content in the suspension. The suspensions were named as2TODA suspension and 2ESD suspension respectively. Both processingagents stabilized the suspensions concentrated to a loading of 55% byweight based on total weight of the nanoparticles in the suspension tofacilitate attrition milling process.

Advantages of using TODA combined with the appropriate capping step areconsidered to be as follows: TODA molecules, with a tail of hydrophilicmulti-ether group and a head of carboxylic acid group, can be chemicallybonded onto the zirconia nanoparticle surfaces by reacting with thesurface hydroxyl groups. The multi-ether tails provide stearic repulsionneeded to keep zirconia nanoparticles well dispersed in water andprevent agglomeration when suspension is concentrated. Compared to ionicprocessing agents, such as ionic amines like tetramethylammoniumhydroxide (TMAH), polyacrylates salts like ammonium polyacrylate, orother commonly used anionic processing agents like triammonium citrate(TAC), which are physically absorbed on nanoparticulate surfaces, thechemically bonded TODA dispersant is better suited to survive theosmotic processing of this disclosure without detaching from thenanoparticle surfaces and escaping through the dialysis membrane tubinginto osmotic solution outside membrane. Furthermore, since TODA isacidic, it shifts the pH value of the suspension to lower values awayfrom its isoelectric point (IEP). On the other hand, adding processingagents such as TMAH and TAC can potentially cause the irreversibleagglomeration of particles since they shift the pH to basic range on theother side of IEP casing gelation often associated with formation ofhard agglomerates. It was also found that gels made from 2TODAsuspensions have higher green body survival rate than 2ESD suspensionswhen no particle interaction strengthening agent (e.g., polymericprocessing agent) was added. It was unexpectedly found that with propercapping procedure the amount of TODA needed to provide well dispersedsuspension was well below saturation limit required to cover the wholesurface of all the nanoparticles in suspension (see FIG. 4).

Any steric, electrosteric or electrostatic processing agent, forexample, 2-[2-(2-methoxyethoxy)ethoxy]acetic acid (TODA),2-(2-methoxyethoxy) acetic acid (also known as 3,6-dioxaheptanoic acidabbreviated as DOHA) or ESD, can be used as long as it allows productionand use (according to examples of this disclosure) of well-dispersedsuspensions and gels at such agent concentrations not exceeding 2.2% byweight based on total weight of the nanoparticles in the suspension andproviding that such dispersant is not increasing pH of the startingnanozirconia suspension to above 5.5 and, for example, not higher than4.5.

In an example, a method of making a gel of the present disclosure (e.g.,a gel of Statement 1) comprises: a) providing an aqueous suspensioncomprising zirconia nanoparticles having an average size of 10 to 30 nmand 95% or more of the zirconia nanoparticles by volume have a size of45 nm or less, where the zirconia nanoparticles are present at less than70% by weight of the aqueous suspension; and b) concentrating theaqueous suspension by removing water from the aqueous suspension with asemipermeable membrane, where the water removal is driven byintrinsically induced pressure or externally imposed pressure, until thesuspension has zirconia nanoparticles present at 70 to 85% by weight ofthe aqueous suspension, whereby the gel is formed. In various examples,the zirconia nanoparticles are present at 50 to 70% by weight of theaqueous suspension or 50 to 60% by weight of the aqueous suspension.

The aqueous suspension can further comprise a processing agent. Theaqueous suspension can comprise varying amounts of one or moreprocessing agents. Unless otherwise indicated, the amount of processingagent(s) in the aqueous suspension is a percentage by weight based onthe total weight of the nanoparticles in an aqueous suspension. Invarious examples, the total amount of the processing agents is presentat 1.5% to 3.3% by weight, including all 0.1% by weight values andranges therebetween, based on the total weight of the nanoparticles inthe aqueous suspension. Examples of aqueous suspensions comprising anelectrostatic colloid stabilizers are referred to herein as a 2ESD or2ESD suspension. At least a portion or all of the processing agent(s)is/are attached via at least one covalent bond to at least a portion ofthe zirconia nanoparticles.

A method of making a gel can comprise attrition milling a startingaqueous suspension. A method of making a gel can comprise attritionmilling a starting aqueous suspension and subjecting the attritionmilled starting aqueous suspension to centrifugation.

Deagglomeration occurs during the attrition milling process, asevidenced by, for example, a drop of viscosity and arising translucencyof the suspension. For example, as described in Comparative Example 1,the viscosity of the 2TODA suspension was typically 50-60 cP beforeattrition milling and fell to 10 to 20 cP after 105 minute attritionmilling at the same shear rate of 2.64 s-1. The milky suspension startedto turn translucent after relatively short attrition milling time. After30 minutes the milled suspension became very noticeably translucent andtranslucency kept increasing until reaching maximum at about 105 minutesof attrition milling. The deagglomeration of particles can also bedirectly observed by Dynamic Light Scattering (DLS) measurements asshown in FIG. 6B. Before attrition milling both suspensions had unimodalor bimodal particle size distribution with the volume average particlesize ranging from 54 to 68 nm for 2TODA suspensions and 36 to 44 nm for2ESD suspensions. After 105 minutes of attrition milling, the largerpeak disappeared if present initially and volume average particle sizefor both suspensions decreased to 18 to 25 nm range. The attritionmilling step is very important to form an agglomeration-free gel,without which the formed gel by osmotic processing was opaque even at aloading less than 70% by weight based on total weight of the suspension.

The most indicative parameter to evaluate the quality of suspension isparticle size distribution by volume weight (volume percentage). Forexample, the expression of D(v)50=100 nm means that 50 vol % ofparticles are smaller than 100 nm. For example, FIG. 13 shows that theattrition milling effectively reduced the particle size for “good”suspensions after 105 minutes. Attrition milling specifically targets tobreak large agglomerates. However, for “bad” suspensions, attritionmilling was not able to reduce particle size or break largeagglomerates.

For example, a method of making a gel can comprises attrition milling astarting aqueous suspension comprising zirconia nanoparticles having anaverage size greater than 50 nm, where the zirconia nanoparticles arepresent at 50% or greater by weight of the starting aqueous suspension,and, optionally, subjecting the attrition milled starting aqueoussuspension to centrifugation, to provide an aqueous suspension (e.g., anaqueous suspension of a) above).

A starting aqueous suspension can comprise zirconia nanoparticles havingan average size greater than 50 nm, where the zirconia nanoparticles arepresent at less than 50% by weight of the starting aqueous suspensionbased on the total weight of the starting aqueous suspension. In thiscase a method further comprises: i) concentrating the starting aqueoussuspension by heating and/or applying sub-ambient pressure to thestarting aqueous solution until the zirconia nanoparticles are presentat 50% or greater by weight; and ii) attrition milling the concentratedstarting aqueous suspension from i) and, optionally, subjecting theattrition milled starting aqueous suspension to centrifugation, toprovide the aqueous suspension of a).

A colloidal stabilizer can be added to the starting aqueous suspensionprior to attrition milling. For example, the starting aqueous suspensionprior to attrition milling further comprises a colloidal stabilizer(e.g., the total amount of the colloidal stabilizer is present at 0.5 to2.5% by weight based on the total weight of zirconia nanoparticles inthe starting aqueous suspension.

A particle interaction strengthening agent can be added to the startingaqueous suspension after attrition milling. For example, a particleinteraction strengthening agent is added to the starting aqueoussuspension after attrition milling (e.g., the particle interactionstrengthening agent is present at 0.1 to 1.5% by weight based on thetotal weight of the nanoparticles in the milled starting aqueoussuspension).

The aqueous suspension can have various pH values. In an example, the pHof the aqueous suspension is 2.5 to 5.5, including all 0.1 pH values andranges therebetween. In an example, the pH of the aqueous suspension is2.5 to 4.5. In various examples, the pH of the aqueous suspension is 5.5or less or 4.5 or less.

Water can be removed from the aqueous suspension using an intrinsicallyinduced pressure. Without intending to be bound by any particulartheory, it is considered that intrinsically induced pressure resultsfrom a thermodynamic driving force or a difference in chemical potential(e.g., resulting from different solute concentrations of two solutionson opposite sides of a semipermeable membrane).

The water removal process (e.g., a portion of or all of the process) canbe carried out with physical agitation (e.g., shaking or stirring) ofthe aqueous suspension. For example, the aqueous suspension isphysically agitated during at least a portion of or all of the waterremoval process. Without intending to be bound by any particular theory,it is considered that physical agitation during water removalhomogenizes the resulting gel. It is desirable the physical agitationresult in minimal or no cavitation (e.g., no observable cavitation) ofthe aqueous suspension. For example, during a portion of or all of thewater removal (e.g., water removal by osmosis) the aqueous suspension isshaken. It is desirable the physical agitation on the aqueous suspension(not just the osmotic solution) can significant improve the homogeneityand cause very minimal cavitation of the suspension as it isconcentrated to form the gel.

Water can be removed from the aqueous suspension using an osmoticprocess. An osmotic process provides intrinsically induced pressure. Forexample, the concentrating in a method is carried out by an osmoticprocess.

FIG. 5 is a diagram of an example of osmotic processing, which isdescribed in detail in Example 1. Nanozirconia suspensions with 55 wt %solid loading (based on total weight of the nanoparticles in thesuspension) were loaded in a semi-permeable membrane tubing, whileleaving ⅓ to ¼ of the tubing unfilled. The loaded tubing was then closedand immersed in a polymer/water osmotic solution in a closed container.The molecular weight of the polymer was chosen to be greater than themolecular weight cut off (MWCO) of the membrane so that the polymermolecules could not penetrate the membrane to contaminate thesuspension. The container was then placed on a shaking table andsubjected to continuous agitation for 16 to 36 hours at 100 to 150 rpm.It is important to keep the tubing not fully filled and keep itcontinuously agitated, so that the suspension tumbles in the tubing andis homogenized during the process. Driven by the osmotic pressure, waterwas drawn from the nanoparticle suspension inside the semi-permeabletubing to the osmotic solution outside, while zirconia nanoparticlescould not pass through due to their bigger size. The suspension was thusconcentrated and later formed a gel. It is evidenced that the osmoticprocessing did not result in the agglomeration of zirconia nanoparticlessince the gel was highly translucent and could be totally re-dispersedin water to form a stable and translucent suspension. The formed gelneeds to be soft, homogenous and easily deformable so that it can beshaped by various kinds of forming methods such as centrifugal casting,CIP, vibra-forming, etc.

The osmotic solution used in the osmotic process can be an aqueouspolymer solution. Various polymers that are soluble in an aqueous medium(e.g., water) can be used. For example, the polymer of the aqueouspolymer solution is selected from the group consisting of polyethyleneglycol (PEG), poly-ethylene-co-propylene glycol, polyethylene imine(PEI), and combinations thereof.

For example, the osmotic process comprises: a) placing the aqueoussuspension comprising zirconia nanoparticles in an enclosure at least aportion of an external surface of which is a semipermeable membrane; b)contacting the enclosure with an osmotic solution such that the osmoticsolution and the aqueous suspension are in fluid contact; c) agitatingthe aqueous suspension; and d) optionally, heating the osmotic solution.

Water can be removed from the aqueous suspension using a tangential flowfiltration process. In this case, the water removal is driven byexternally imposed pressure. For example, the concentrating in a methodis carried out using a tangential flow filtration process.

For example, the tangential flow filtration process comprises: a)flowing the aqueous suspension comprising zirconia nanoparticles througha channel at least a portion an external surface of which is thesemipermeable membrane; b) repeating a) until the a desired amount ofwater is removed from the aqueous suspension comprising zirconiananoparticles; and c) optionally, heating the aqueous suspensioncomprising zirconia nanoparticles.

The semipermeable membrane in the tangential flow filtration process canbe contacted with an osmotic solution. The osmotic solution can be anosmotic solution described herein.

Externally imposed pressure can be provided by mechanical means. Forexample, externally imposed pressure can be provided by a pump (e.g., amechanical pump). In various examples, an externally imposed pressure of5 to 200 psi, including all integer psi values and ranges therebetween.

In an aspect, the present disclosure provides methods of making zirconiagreen bodies. The methods are based on the removal of water (e.g.,non-equilibrium water) from a gel (e.g., a gel that has been shaped intoa desired shape).

For example, a method of forming a green body of the present disclosure(e.g., a green body of Statement 22) comprises: a) providing a gel ofthe present disclosure (e.g., a gel of Statement 1); b) shaping the gelinto a desired shape; and c) removing water from the shaped gel, wherebythe zirconia green body having 2% to 5% water, including all 0.1% valuesand ranges therebetween, based on the weight of the zirconia green bodyis formed.

The gel can be shaped. For example, the shaping of the gel comprisesshaping the gel into an isotropically enlarged, uniform shape. Theshaping can be carried out using centrifugal casting, cold isostaticpressing (CIP), vibra forming, or injection molding.

Centrifugal casting and cold isostatic pressing (CIP) can be used toform desired shapes from gels, for example, as described in Example 1and Example 13, respectively. In order to form a green body to survivethe drying process, it is desirable that the shaped gel has minimumamount of defects (bubbles, voids and surface roughness, etc.). Gelformability ranking is thus introduced as a qualitative measure of thegiven gel capacity to form such shapes, which is largely determined bythe gel yield stress and viscosity.

Gel yield stress and viscosity were characterized by rheometrymeasurements and correlated with its formability ranking, which isdescribed in Example 28. Both gels made from 2TODA suspensions (Example1, 2A and 5A) and gels made from 2ESD suspensions (Example 23, 8A and11A) were measured. It was found that the gels, except for gel ofExample 23, were initially deformed like an elastomer until a yieldstress was reached. Further increasing shear stress results in the flowbehavior of the gels, as shown in FIGS. 7A and 7B. The gel of Example 23is too rigid to yield even at the maximum shear stress of 10,000 Pa.When shear stress exceeded the yield stress, gels showed a strongshear-thinning behavior as shown in FIGS. 8A and 8B. These measurementresults were summarized in Table 1A, where the assignment ofsoftness/rigidity, ranging from very soft, soft, semi-soft, semi-rigidto rigid, was also listed. Based on these experimental results, aqualitative assessment of gel formability ranking was established basedon the gel viscosity at yield point, as shown in Table 1B. Gels withviscosity less than 4×10⁹ mPa·s are assigned to be the most formable(Level 5), while gels with viscosity of 4 to 8×10⁹ mPa·s and 8 to 12×10⁹mPa·s are also formable but with lower formability levels (Level 3-4 and1-2 respectively). The higher the gel formability level, the less likelythat bubble/voids would be trapped in the body when they are formed.Furthermore, the top surface of the formed body is smoother when the gelformability is higher, as shown in FIG. 9 where centrifugal casting wasused.

TABLE 1A Viscosity and yield stress measured with Kinexus Ultra+Rotational Rheometer by Malvern (Example 28) Viscosity Gel at yieldloading, Yield point wt % stress, (×10⁹ Softness/ Gel Processing agent(vol%) ×10³ Pa mPa · s) Rigidity gel of 2TODA 81.3 4.1 4.6 soft EX1(41.6) gel of 1% 77.4 3.0 2.2 very soft EX2A PEG35K (36.0) gel of 0.1%78.8 6.6 9.3 semi-soft EX5A PEG35K + (37.9) 0.05% PVA gel of 2ESD80.2 >10 >46.3 semi-rigid EX23 (39.9) to rigid gel of 1% 75.8 5.1 8.7soft to EX8A PEG35K (33.9) semi-soft gel of 0.1% 75.3 5.2 8.7 soft toEX11A PEG35K + (33.3) semi-soft 0.05% PVA

TABLE 1B Gel softness/rigidity and formability correlation to gelviscosity at yield point Softness/ Viscosity at yield Rigidity*Formability point, ×10⁹ mPa · s Very soft Level 5: Best formability withsmooth <4 top surface and the least likelihood of trapped bubbles SoftLevel 3-4: Formable but top surface 4-8 could be a little roughSemi-soft Level 1-2: Formable with rough top  8-12 surface, bubbles morelikely trapped Semi-rigid Level 0: Not soft enough (too hard) 12-50 tobe formable Rigid Semi solid to solid >50 1 mPa · s = 1 cP = 0.01 P =0.001 Pa · s

A semi-rigid gel (viscosity 12 to 50×10⁹ mPa·s) or rigid gel (viscositygreater than 50×10⁹ mPa·s) are considered to be non-formable. These gelsare either not soft enough to form a shape with the above-mentionedforming methods, or just too solid (not plastic, stiff, i.e., rigid) toeven deform.

The solid loading of the gel is a significant major factor affecting gelformability and can be controlled by osmotic time. However, otherfactors, such as the type of processing agent(s) added in thesuspension, can also have a significant impact on formability.

The water can be removed by holding a gel (e.g., a shaped gel) in acontrolled-humidity and controlled-temperature environment. In variousexamples, the humidity of the controlled-humidity environment is 10 to90%, including all integer % values and ranges therebetween, and thetemperature is 20 to 50° C., including all integer ° C. values andranges therebetween.

The water can be removed by holding a gel (e.g., a shaped gel) inmultiple controlled-humidity and controlled-temperature environmentshaving different humidity and temperature. For example, the removing iscarried out by holding the shaped gel in two or more differentcontrolled-humidity environments, where the humidity in each of theindividual environments is 10 to 90% and different than the otherindividual environments and is decreased relative to the previousenvironment in which the gel was held. For example, a first environmenthas a humidity of 70 to 90% and a second environment has a humidity of10 to 30%.

For example, the removing is carried out by: i) holding the shaped gelin the first environment for 1 to 10 days (e.g., 1, 2, 3, 4, 5, 6, 7, 8,9, or 10 days, or any 0.1 day value between 1 and 10 days; and ii)subsequently holding the shaped gel from i) in two or more otherenvironments for 1 to 10 days, where the humidity in each of the otherenvironments is from 30 to 90% and is decreased relative to the previousenvironment in which the gel was held and the last environment has ahumidity of 10 to 30%.

In an aspect, the present disclosure provides methods of makingpre-sintered ceramic bodies. The methods are based on heating a zirconiagreen body to remove organic materials from the zirconia green bodies.

For example, burning out of organics of green bodies of this disclosurecan be carried out at the rates from, for example, 0.1° C./min to 10°C./min and temperatures from 500° C. to 800° C. with or without short orlong isothermal holds at the temperatures associated with mass lossand/or exothermic activity. Typically slower heating rates may help toeliminate at least some of the isothermal holds while faster heatingrates may require a few isothermal holds at the temperatures associatedwith mass loss and/or exothermic activity including low temperatureisothermal holds (less than 150° C.) for elimination of the absorbedwater. Pre-sintering can be carried out concurrently or separately fromorganic burn out, in one or multiple separate heating cycles carried outin one or different furnaces including environmental chambers, vacuumfurnaces or furnaces able to operate at partial pressures. In cases whenremoval of organics is overlapping with pre-sintering, which is morenoticeable at higher starting green densities and finer nanoparticlesizes wherein organics burn-out and shrinkage due to interparticlenecking cannot be easily deconvoluted, the resulting “pre-sinteredbrown” bodies are still called brown bodies despite their noticeableshrinkage. The latter can happen when green density is above 55% oftheoretical density, and average particle size in the startingnanozirconia green body is less than 25 nm; and organic burn out iscarried to the temperatures higher than 550° C. Organic burn out ofcommercially relevant size blanks (greater than 10 mm in thickness) andlarge green-milled restorations is done much slower than small samplesbut once organics (e.g., processing agents) are fully removedpre-sintering and sintering to full density can be carried out at rateshigher than 10° C./min.

A pre-sintered ceramic body (e.g., a pre-sintered ceramic body ofStatement 44) can be made by heating a zirconia green body of thepresent disclosure (e.g., a zirconia green body of Statement 22) at atemperature of 400 to 1000° C., including all integer ° C. values andranges therebetween. The heating can be carried out at a heating rate of0.1 to 10° C./min, including all 0.1° C./min values and rangestherebetween. For example, a method of making a pre-sintered ceramicbody of the present disclosure (e.g., a pre-sintered ceramic body ofStatement 44), comprises heating a zirconia green body of the presentdisclosure (e.g., a zirconia green body of Statement 22) at atemperature of 400 to 1000° C., where the zirconia green body may beheated at a rate of 0.1 to 10° C./min.

In an aspect, the present disclosure provides methods of making zirconiadental ceramics. The methods are based on heating a pre-sintered ceramicbody as described herein. In various examples, a method of making azirconia dental ceramic material comprises heating a pre-sinteredceramic body as described herein.

In an aspect, the present disclosure provides methods of making dentalarticles. The methods are based on shaping and heating a pre-sinteredceramic body or zirconia green body.

A dental article of the present disclosure (e.g., a dental article ofStatement 55) can be made by first shaping comprising shaping a greenbody of the present disclosure (e.g., a green body of Statement 22) or apre-sintered ceramic body of the present disclosure (e.g., apre-sintered ceramic body of Statement 44). The shaping can be carriedout by methods known in the art. For example, the shaping is carried outby CAD/CAM, Low Pressure Injection Molding (LPIM), or dental heatpressing. Then the shaped green body or shaped pre-sintered ceramic bodyis heated at a temperature of 1000 to 1200° C., to form the dentalrestoration.

For example, a method of making a dental article of the presentdisclosure (e.g., a dental article of Statement 55) comprises shaping(e.g., by CAD/CAM, LPIM, or dental heat pressing) a green body of thepresent disclosure (e.g., a green body of Statement 22) or apre-sintered ceramic body of the present disclosure (e.g., apre-sintered ceramic body of Statement 44) and heating the shaped greenbody or shaped pre-sintered ceramic body at a temperature of 1000 to1200° C., whereby the dental restoration is formed. For example, theshaping of a zirconia green body of the present disclosure (e.g., azirconia green body of Statement 22) or a pre-sintered ceramic body ofthe present disclosure (e.g. a pre-sintered ceramic body of Statement44) is carried out using CAD/CAM, LPIM, or dental heat pressing.

The steps of the methods described in the various embodiments andexamples disclosed herein are sufficient to produce materials of presentdisclosure (e.g., gels, zirconia green bodies, pre-sintered ceramicbodies, zirconia ceramics, and dental restorations) of the presentdisclosure. Thus, in an embodiment, a particular method consistsessentially of a combination of the steps of the method disclosedherein. In another embodiment, a particular method consists of suchsteps.

In the following Statements, various examples of the compositions andmethods of the present disclosure are described:

Statement 1. A gel comprising a plurality of zirconia nanoparticles andwater, where the zirconia nanoparticles have an average size of 10 to 30nm, 95% or more of the zirconia nanoparticles by volume have a size of45 nm or less, and the gel is a formable gel (e.g., formable into thedesired shape (e.g., by centrifugal force, vibration, pressure, or acombination thereof) and capable of losing water in acontrolled-humidity and controlled-temperature environment withoutcracking while maintaining the shape integrity, whereby a zirconia greenbody having 2% to 5% water based on the weight of the zirconia greenbody is formed).

Statement 2. A gel according to Statement 1, where: i) 99% ofnanoparticles by volume have a size less than 60 nm±10 nm; ii) 95% ofnanoparticles by volume have a size less than 40 nm±5 nm; iii) 50% ofnanoparticles by volume have a size less than 20 nm±5 nm; and iv.) 5% ofnanoparticles by volume have a size less than 12 nm±3 nm.

Statement 3. A gel according to any one of Statements 1 and/or 2, where95% or greater by volume of the zirconia nanoparticles comprise 1 to 5crystallites.

Statement 4. A gel according to any one or more of the precedingStatements, where the zirconia nanoparticles are present at 70 to 85% byweight based on the total weight of the gel.

Statement 5. A gel according to any one or more of Statements 1 to 4,where the zirconia nanoparticles are present at 28 to 48% by volumebased on the total volume of the gel.

Statement 6. A gel according to any one or more of the precedingStatements, where the gel further comprises one or more processingagents and/or one or more additives (e.g., oxides and colorants (e.g.,coloring oxides and coloring ions), compounds comprising rare earthelements and/or comprising lanthanum group elements, and combinationsthereof).

Statement 7. A gel of Statement 6, where the processing agent isselected from the group consisting of colloid stabilizers, particleinteraction strengthening agents, and combinations thereof.

Statement 8. A gel of Statement 7, where the particle interactionstrengthening agent is selected from the group consisting ofpolyethylene glycol (PEG), polyethylene oxide (PEO), polyvinylalcohol(PVA), methyl cellulose, polyacrylic acid, dextrin,poly-ethylene-co-propylene-glycol, and combinations thereof.

Statement 9. A gel of Statement 7, where the colloid stabilizers areselected from dispersants, protective colloids, and combinationsthereof.

Statement 10. A gel of Statement 7, where the colloid stabilizer isselected from the group consisting of organocarboxylic acids (e.g.,polyoxocarboxylic acids such as, for example, CH₃—[O—(CH₂CH₂)]_(x)CO₂Hand salts thereof, where x is 1 to 120, 1 to 50, or 1 to 30) and saltsthereof, amino acids and salts thereof, organoamines and ammonium saltsthereof, organoalcohols and organosilanes.

Statement 11. A gel of Statement 10, where the colloid stabilizers areselected from the group consisting of2-[2-(2-methoxyethoxy)ethoxy]acetic acid (TODA),2-(2-methoxyethoxy)acetic acid (DOHA),O-(2-carboxyethyl)-O′-methyl-undecaethylene glycol, methoxypolyethyleneglycol propionic acid (e.g., having a molecular weight of 5,000),3,6,9-trioxaundecanedioic acid, polyacrylic acid, bicine, dodecyl amine,tetradecyl methyl amine, cetyl trimethyl ammonium bromide (CTAB),ammonium polyacrylate, polyethylene glycol dodecyl ether,trimethoxy(propyl)silane,2-[(acetoxy(polyethyleneoxy)propyl]triethoxysilane and2-[methoxy(triethyleneoxy)propyl]trimethoxysilane, and combinationsthereof.

Statement 12. A gel of Statement 6, where the processing agent is 2% byweight TODA and/or ESD and 0.1% by weight PEG35K and 0.05% by weight PVAor 0.5% percent by weight PEG35k and 0.05% by weight PVA, where thepercent by weight values are based on the total weight of thenanoparticles in the gel.

Statement 13. A gel according to any one or more of Statements 6 to 11,where the amount of the processing agent is 1.5 to 3.3% by weight basedon the total weight of the nanoparticles in the gel.

Statement 14. A gel according to any one or more of Statements 6 to 13,where at least a portion of the processing agent is attached via atleast one covalent bond to at least a portion of the zirconiananoparticles.

Statement 15. A gel according to any one or more of the precedingStatements, where the gel is translucent and has a transmittance at 560nm wavelength of 60 to 80% for a 1 to 2 mm thick sample of the gel.

Statement 16. A gel according to any one or more of the precedingStatements, where the gel has an opalescence of 20 to 30 for a 1 to 2 mmthick sample of the gel.

Statement 17. A gel according to any one of the preceding Statements,where the gel is redispersible in an aqueous medium.

Statement 18. A gel according to any one or more of the precedingStatements, where the gel exhibits a viscosity at yield point of 1×10⁹to 12×10⁹ mPa·s.

Statement 19. A gel according to any one or more of the precedingStatements, where the gel exhibits a yield stress of 1×10³ to 9×10³ Pa.

Statement 20. A gel according to any one or more of the precedingStatements, where in the zirconia nanoparticles are in a tetragonalphase.

Statement 21. A gel according to any one or more of Statements 1 to 19,where 10% or less of the zirconia nanoparticles are in a cubic and/or amonoclinic phase.

Statement 22. A zirconia green body comprising a plurality of zirconiananoparticles, where the zirconia nanoparticles have an average size of10 to 30 nm and 95% or more of the zirconia nanoparticles by volume havea size of 45 nm or less.

Statement 23. A zirconia green body, of Statement 22, where: i) 99% ofnanoparticles by volume have a size less than 60 nm±10 nm; ii) 95% ofnanoparticles by volume have a size less than 40 nm±5 nm; iii) 50% ofnanoparticles by volume have a size less than 20 nm±5 nm; and iv.) 5% ofnanoparticles by volume have a size less than 12 nm±3 nm.

Statement 24. A zirconia green body according to any one or more ofStatements 22 to 24, where the zirconia green body is porous.

Statement 25. A zirconia green body of Statement 24, where the zirconiagreen body comprises pores having a size of 3 to 10 nm and at least aportion of the pores are interconnected.

Statement 26. A zirconia green body according to any one or more ofStatements 22 to 25, where the zirconia green body has a surface area of40 to 80 m²/g.

Statement 27. A zirconia green body according to any one or more ofStatements 22 to 26, where the zirconia green body has a density of 50to 70% of the zirconium dioxide theoretical density.

Statement 28. A zirconia green body according to any one or more ofStatements 22 to 27, where in the zirconia nanoparticles are in atetragonal phase.

Statement 29. A zirconia green body according to any one or more ofStatements 22 to 27, where 10% or less of the zirconia nanoparticles arein a cubic and/or a monoclinic phase.

Statement 30. A zirconia green body according to any one or more ofStatements 22 to 29, where the zirconia green body further comprises 2to 5% by weight based on the total weight of the zirconia green body atequilibrium with the ambient environment at room temperature.

Statement 31. A zirconia green body according to any one or more ofStatements 22 to 30, where the zirconia green body further comprises aprocessing agent.

Statement 32. A zirconia green body of Statement 31, where theprocessing agent is selected from the group consisting of colloidstabilizers, particle interaction strengthening agents, and combinationsthereof.

Statement 33. A zirconia green body of Statement 32, where the particleinteraction strengthening agent is selected from the group consisting ofpolyethylene glycol (PEG), polyethylene oxide (PEO), polyvinylalcohol(PVA), methyl cellulose, polyacrylic acid, dextrin,poly-ethylene-co-propylene-glycol, and combinations thereof.

Statement 34. A zirconia green body according to any one of Statements32 and/or 33, where the colloid stabilizers are selected fromdispersants, protective colloids, and combinations thereof.

Statement 35. A zirconia green body of Statement 32, where the colloidstabilizer is selected from the group consisting of organocarboxylicacids (e.g., polyoxocarboxylic acids such as, for example,CH₃—[O—(CH₂CH₂)]_(x)CO₂H and salts thereof, where x is 1 to 120, 1 to50, or 1 to 30), amino acids, organoamines, organoalcohols andorganosilanes.

Statement 36. A zirconia green body of Statement 35, where thedispersant selected from the group consisting of2-[2-(2-methoxyethoxy)ethoxy]acetic acid (TODA),2-(2-methoxyethoxy)acetic acid (DOHA),O-(2-carboxyethyl)-O′-methyl-undecaethylene glycol, methoxypolyethyleneglycol propionic acid (e.g., having a molecular weight of 5,000),3,6,9-trioxaundecanedioic acid, polyacrylic acid, bicine, dodecyl amine,tetradecyl methyl amine, cetyl trimethyl ammonium bromide (CTAB),ammonium polyacrylate, polyethylene glycol dodecyl ether,trimethoxy(propyl)silane,2-[(acetoxy(polyethyleneoxy)propyl]triethoxysilane and2-[methoxy(triethyleneoxy)propyl]trimethoxysilane, and combinationsthereof.

Statement 37. A zirconia green body according to any one or more ofStatements 31 to 36, where the processing agent is 2% by weight TODAand/or ESD and 0.1% by weight PEG35K and 0.05% by weight PVA or 0.5%percent by weight PEG35k and 0.05% by weight PVA, where the percent byweight values are based on the total weight of the nanoparticles in thezirconia green body.

Statement 38. A zirconia green body according to any one or more ofStatements 31 to 37, where the amount of the processing agent is 1.5 to3.3% by weight based on the total weight of the nanoparticles in thezirconia green body.

Statement 39. A zirconia green body according to any one or more ofStatements 22 to 38, where the zirconia green body is translucent andhas a transmittance at 560 nm wavelength of 50 to 70% for a 1 to 2 mmthick sample of the zirconia green body.

Statement 40. A zirconia green body according to any one or more ofStatements 22 to 39, where the zirconia green body has an opalescence of20 to 30 for a 1 to 2 mm thick sample of the zirconia green body.

Statement 41. A zirconia green body of one or more of Statements 22 to40, where the zirconia green body exhibits a Vickers hardness of 35 to70 kg/mm².

Statement 42. A zirconia green body according to any one or more ofStatements 22 to 41, where the zirconia green body has a dimension of 10to 30 mm in a direction perpendicular to the longest dimension of thezirconia green body (thickness).

Statement 43. A zirconia green body according to any one or more ofStatements 22 to 42, where the zirconia green body has a longestdimension of 15 to 100 mm.

Statement 44. A pre-sintered ceramic body comprising a plurality ofzirconia nanoparticles, where the pre-sintered ceramic body is porous,the pre-sintered ceramic body is translucent and has a transmittance at560 nm wavelength of 40 to 60% for a 1 to 2 mm thick sample of thepre-sintered ceramic body, and the pre-sintered ceramic body has anopalescence of 25 to 35 for a 1 to 2 mm thick sample of the pre-sinteredceramic body.

Statement 45. A pre-sintered ceramic body of Statement 44, where thepre-sintered ceramic body has a density of 50 to 70% of the zirconiumdioxide theoretical density.

Statement 46. A pre-sintered ceramic body of one of Statements 44 and/or45, where in the zirconia nanoparticles are in a tetragonal phase.

Statement 47. A pre-sintered ceramic body according to any one ofStatements 44 and/or 45, where 10% or less of the zirconia nanoparticlesare in cubic and/or monoclinic phase.

Statement 48. A pre-sintered ceramic body according to any one or moreof Statements 44 to 47, where the pre-sintered ceramic body has adimension of 10 to 30 mm in a direction perpendicular to the longestdimension of the pre-sintered ceramic body (thickness).

Statement 49. A pre-sintered ceramic body according to any one or moreof Statements 44 to 48, where the pre-sintered ceramic body has alongest dimension of 15 to 100 mm.

Statement 50. A zirconia dental ceramic, where at least 95% of allgrains by volume have a size of 10 nm to 300 nm and the average grainsize is 40 nm to 150 nm, the density of the zirconia dental ceramic hasa density that is at least 99.5% of zirconium dioxide theoreticaldensity, and the zirconia dental ceramic is opalescent.

Statement 51. A zirconia dental ceramic of Statement 50, where thezirconia dental ceramic is translucent and has a transmittance at 560 nmwavelength of 25 to 50% for a 1 to 2 mm thick sample of the zirconiadental ceramic.

Statement 52. A zirconia dental ceramic according to any one ofStatements 50 and/or 51, where the zirconia dental ceramic has anopalescence of 9 or greater for a 1 to 2 mm thick sample of the zirconiadental ceramic.

Statement 53. A zirconia dental ceramic according to any one or more ofStatements 50 to 52, where the zirconia dental ceramic has an averageflexural strength of 1200 MPa or greater.

Statement 54. A zirconia dental ceramic according to any one or more ofStatements 50 to 53, where the zirconia dental ceramic has an averagetensile strength of 500 MPa or greater.

Statement 55. A dental article formed from a zirconia dental ceramic ofthe present disclosure (e.g., the zirconia dental ceramic of Statement50) or a formable gel of the present disclosure (e.g., a formable gel ofStatement 1).

Statement 56. A dental article of Statement 55, where the dental articleis a blank or smart blank.

Statement 57. A dental article of Statement 55, where the dental articleis a dental restoration.

Statement 58. A dental article of Statement 57, where the dentalrestoration is selected from full-contour FPDs (fixed partial dentures),bridges, implant bridges, multi-unit frameworks, abutments, crowns,partial crowns, veneers, inlays, onlays, orthodontic retainers, spacemaintainers, tooth replacement appliances, splints, dentures, posts,teeth, jackets, facings, facets, implants, cylinders, and connectors.

Statement 59. A method of making a gel of the present disclosure (e.g.,a gel of Statement 1) comprising: a) providing an aqueous suspensioncomprising zirconia nanoparticles having an average size of 10 to 30 nmand 95% or more of the zirconia nanoparticles by volume have a size of45 nm or less, where the zirconia nanoparticles are present at less than70% by weight of the aqueous suspension; and b) concentrating theaqueous suspension by removing water from the aqueous suspension with asemipermeable membrane, where the water removal is driven byintrinsically induced pressure or externally imposed pressure, until thesuspension has zirconia nanoparticles present at 70 to 85% by weight ofthe aqueous suspension, whereby the gel is formed.

Statement 60. A method of Statement 59, where the aqueous suspensionfurther comprises a processing agent.

Statement 61. A method according to Statement 60, where the total amountof the processing agents is present at 1.5% to 3.3% by weight based onthe total weight of the nanoparticles in the aqueous suspension.

Statement 62. A method according to any one of Statements 59 to 61,where at least a portion of the processing agent is attached via atleast one covalent bond to at least a portion of the zirconiananoparticles.

Statement 63. A method of Statement 59, further comprising: attritionmilling a starting aqueous suspension comprising zirconia nanoparticleshaving an average size greater than 50 nm, where the zirconiananoparticles are present at 50% or greater by weight of the startingaqueous suspension based on the total weight of the starting aqueoussuspension, and, optionally, subjecting the attrition milled startingaqueous suspension to centrifugation, to provide the aqueous suspensionof a).

Statement 64. A method of Statement 63, where the starting aqueoussuspension prior to attrition milling further comprises a colloidalstabilizer.

Statement 65. A method of Statement 64, where the total amount of thecolloidal stabilizer is present at 0.5 to 2.5% by weight based on thetotal weight of zirconia nanoparticles in the starting aqueoussuspension.

Statement 66. A method according to any one or more of Statements 63 to65, where a particle interaction strengthening agent is added to thestarting aqueous suspension after attrition milling.

Statement 67. A method according to Statement 66, where the total amountof the particle interaction strengthening agent is present at 0.1 to1.5% by weight based on the total weight of the nanoparticles in themilled starting aqueous suspension.

Statement 68. A method according to Statement 59, where a startingaqueous suspension comprises zirconia nanoparticles having an averagesize greater than 50 nm, where the zirconia nanoparticles are present atless than 50% by weight of the starting aqueous suspension based on thetotal weight of the starting aqueous suspension and the method furthercomprises: i) concentrating the starting aqueous suspension by heatingand/or applying sub-ambient pressure to the starting aqueous solutionuntil the zirconia nanoparticles are present at 50% or greater by weightbased on the total weight of the starting aqueous suspension; and ii)attrition milling the concentrated starting aqueous suspension from i)and, optionally, subjecting the attrition milled starting aqueoussuspension to centrifugation, to provide the aqueous suspension of a).

Statement 69. A method according to Statement 68, where a colloidalstabilizer is added to the starting aqueous suspension prior to theconcentration of the starting aqueous suspension.

Statement 70. A method according to Statement 69, where the total amountof the colloidal stabilizer is present at 0.5 to 2.5% by weight based onthe total weight of the nanoparticles in the starting aqueoussuspension.

Statement 71. A method according to any one or more Statements 68 to 70,where a particle interaction strengthening agent is added to thestarting aqueous suspension after attrition milling.

Statement 72. A method according to Statement 71, where the total amountof the particle interaction strengthening agent is present at 0.1 to1.5% by weight based on the total weight of the nanoparticles in themilled starting aqueous suspension.

Statement 73. A method according to any one or more of Statements 59 to72, where the concentrating is carried out by an osmotic process.

Statement 74. A method according to Statement 73, where the osmoticsolution used in the osmotic process is an aqueous polymer solution.

Statement 75. A method according to Statement 74, where the polymer ofthe aqueous polymer solution is selected from the group consisting ofpolyethylene glycol (PEG), poly-ethylene-co-propylene glycol,polyethylene imine (PEI), and combinations thereof.

Statement 76. A method according to any one or more of Statements 73 to75, where the osmotic process is carried out with physical agitation ofthe aqueous suspension.

Statement 77. A method according to Statement 73, where the osmoticprocess comprises: a) placing the aqueous suspension comprising zirconiananoparticles in an enclosure at least a portion of an external surfaceof which is a semipermeable membrane; b) contacting the enclosure withan osmotic solution such that the osmotic solution and the aqueoussuspension are in fluid contact; c) agitating the aqueous suspension;and d) optionally, heating the osmotic solution.

Statement 78. A method according to Statement 59, where theconcentrating is carried out by a tangential flow filtration process.

Statement 79. A method according to Statement 78, where the tangentialflow filtration process comprises: a) flowing the aqueous suspensioncomprising zirconia nanoparticles through a channel at least a portionan external surface of which is the semipermeable membrane; b) repeatinga) until the a desired amount of water is removed from the aqueoussuspension comprising zirconia nanoparticles; and c) optionally, heatingthe aqueous suspension comprising zirconia nanoparticles.

Statement 80. A method according to Statement 78, where thesemipermeable membrane is contacted with an osmotic solution.

Statement 81. A method of forming a green body according to the presentdisclosure (e.g., a green body according to Statement 22) comprising: a)providing a gel of the present disclosure (e.g., a gel of Statement 1);b) shaping the gel into a desired shape; and c) removing water from theshaped gel, whereby the zirconia green body having 2% to 5% water basedon the weight of the zirconia green body is formed.

Statement 82. A method according to Statement 81, where the shapingcomprises shaping the gel into an isotropically enlarged, uniform shape.

Statement 83. A method according to any one of Statements 81 and/or 82,where the removing is carried out by holding the shaped gel in acontrolled-humidity and controlled-temperature environment.

Statement 84. A method according to Statement 83, where the humidity is10 to 90% and the temperature is 20 to 50° C.

Statement 85. A method according to any one of Statements 81 and/or 82,where the removing is carried out by holding the shaped gel in two ormore different controlled-humidity environments, where the humidity ineach of the individual environments is 10 to 90% and different than theother individual environments and is decreased relative to the previousenvironment in which the gel was held.

Statement 86. A method according to Statement 85, where a firstenvironment has a humidity of 70 to 90% and a second environment has ahumidity of 10 to 30%.

Statement 87. A method according to Statement 86, where the removing iscarried out by: i) holding the shaped gel in the first environment for 1to 10 days; and ii) subsequently holding the shaped gel from i) in twoor more other environments for 1 to 10 days, where the humidity in eachof the other environments is from 30 to 90% and is decreased relative tothe previous environment in which the gel was held and the lastenvironment has a humidity of 10 to 30%.

Statement 88. A method of making a pre-sintered ceramic body of thepresent disclosure (e.g., a pre-sintered ceramic body according toStatement 44), comprising heating a zirconia green body of the presentdisclosure (e.g., a zirconia green body according to Statement 22) to atemperature of 400 to 1000° C.

Statement 89. A method according to Statement 88, where the zirconiagreen body is heated at a rate of 0.1 to 10° C./min.

Statement 90. A method of making a dental article of the presentdisclosure (e.g., a dental article according to Statement 55) comprisingshaping a green body of the present disclosure (e.g., a green bodyaccording to Statement 22) or a pre-sintered ceramic body of the presentdisclosure (e.g., a pre-sintered ceramic body according to Statement 44)and heating the shaped green body or shaped pre-sintered ceramic body toa temperature according to 1000 to 1200° C., whereby the dentalrestoration is formed.

Statement 91. A method according to Statement 90, where the shaping of azirconia green body of the present disclosure (e.g., a zirconia greenbody according to Statement 22) or a pre-sintered ceramic body of thepresent disclosure (e.g. a pre-sintered ceramic body according toStatement 44) is carried out using CAD/CAM, LPIM, or dental heatpressing.

The following examples are presented to illustrate the presentdisclosure. They are not intended to limiting in any matter.

Comparative Example 1

This example provides a description of comparison of drop casting priorart zirconia suspensions and nanozirconia suspensions of the presentdisclosure.

2 kg of 22 wt % (4.4 vol %) aqueous suspensions of yttria (3 mol %)stabilized zirconia nanoparticulate, containing 0 to 0.3% alumina (e.g.,0.25% alumina) by weight to solid zirconia, were stabilized by adding 2%TODA (obtained from Euticals GmbH) dispersants (2TODA) by weight tosolid zirconia and surface modified at elevated temperature of 60° C.for 1 hour with constant stirring. The pH of such stabilized suspensionwas 2.5. The suspension was then concentrated from 22 wt % to 55 wt %(16.7 vol %) of solid loading with an Ika RV10 vacuum evaporator at 40°C. and 40 mbar for about 4 hours. The concentrated suspension was thenattrition milled with 50 μm diameter yttria-stabilized zirconia beads at3000 rpm rotation speed for 105 minutes, using Netzsch MiniCer attritionmilling machine. Dynamic Light Scattering (DLS) measurement (Nano-ZS byMalvern) was used to monitor the deagglomeration of particles during theattrition milling process, as shown in FIG. 6A.

Cylindrical PTFE molds of 18 mm to 32 mm in diameter and 10 mm in heightwere prepared as it was previously established that it is impossible toproduce viable drop cast samples much thicker than 7-8 mm even in greenstate from suspensions processed as above. From 5 to 15 g of slurry waspoured into each mold depending on the desired final thickness. Thenmolds with suspension were put into an environmental chamber for curingand drying. For the first 72˜120 hours, the humidity was above 85% andtemperature was about 25° C. The drying time was determined by thethickness of the samples. The thicker samples took a longer time to drywithout generating cracks. Then environmental humidity was decreasedgradually to about 20% in 168 hours. The as-formed green bodies were˜49% of theoretical density. Light transmittance was 58% for 2 mm thickgreen body at 560 nm. Dried green bodies were subjected to organic burnout cycle in a burn-out furnace (Vulcan 3-550) by heating at a rate of0.1° C./min to 240° C. and then 0.5° C./min to 550° C. or 700° C. andholding for 2 hours at that temperature. The obtained “brown” bodies(also having high light transmittance of about 50% at 560 nm for 1.8-2.0mm thick samples) were then sintered in a dental furnace or NaberthermHTC 08/16 furnace at a ramp rate of 10° C./min to 1100° C., hold for 2hours and then furnace cooled to idle temperature or to roomtemperature, respectively. After sintering, the disk samples were from12 to 23 mm in diameter and 1.5 mm in thickness.

The samples were then ground and polished down to thickness of 1.0 mm,to prepare samples for optical (transmittance and opalescence)measurements and 1.0-1.2 mm for biaxial flexural strength measurements.The measurement results are presented in Table 2 and compared with thoseof other examples. The number of sample size is 3 for opticalmeasurements and at least 10 for biaxial flexural strength measurements.

TABLE 2 Physical properties of gel-formed articles. ComparativeComparative Example Example 5A: Example 1: Example 2: 2A: Osmotic gel,Drop cast, Drop cast, Osmotic gel, with 0.1% no proces- with 1.0% with1.0% PEG35K + sing agent PEG35k PEG35k 0.05% PVA Sintering 1100° C./2 h1100° C./2 h 1100° C./2 h 1100° C./2 h condition 10° C./min 10° C./min3° C./min 3° C./min Relative 99.78 ± 0.03 99.70 99.82 ± 0.07 99.87 ±0.06 density, % In-line  4.1 ± 0.6 9.0  2.5 ± 0.8  2.5 ± 0.1Transmission at 560 nm, % All forwarding 39.7 ± 0.4 40.6 38.3 ± 1.2 37.9± 0.2 T at 560 nm, % Opalescence 17.4 ± 0.2 18.2 14.6 ± 0.9 15.2 ± 0.2Grain size, nm  90 ± 10 85 ± 11  92 ± 15 N/A Biaxial 1900 ± 156 N/A N/A1807 ± 182 strength, MPa (10, 1599, (11, 1585, (# of samples, 2047)2123) min, max)

Comparative Example 2

This example provides a description of comparison of drop casting priorart zirconia suspensions and nanozirconia suspensions of the presentdisclosure.

The suspension preparation, concentration and attrition-milling stepswere identical to Comparative Example 1. After attrition milling andbefore mold casting, 1.0% by weight to solid zirconia of polyethyleneglycol with number average molecular weight of 35,000 (PEG35K, obtainedfrom SigmaAldrich) was admixed to the suspension and homogenized onshaking table. Identical mold casting, drying, organic burn out,sintering, grinding and polishing steps were then conducted as describedin Comparative Example 1. The transmittance and opalescence measurementswere then performed and the results are shown and compared in Table 2.

Comparative Example 3

This example provides a description of good starting suspensions and badstarting suspensions before and after attrition milling.

FIGS. 11A, 11B, 12A, and 12B show comparison of two suspensions of asimilar solid loading of about 55 wt % and 0.5 wt %: a typicalnanosuspension meeting the requirements of this disclosure (“good”suspension in FIGS. 11A, 11B, 12A and 12B) and unacceptably agglomeratedbatch of nanosuspension not processable by methods described herein(“bad” suspension in FIGS. 11A, 11B, 12A, and 12B). After TODA surfacemodification and concentration to 55 wt %, both suspensions wereattrition milled for up to 2 hours. As it can be clearly seen, the goodsuspension gradually turned to be translucent from milky appearanceduring attrition milling, while the bad suspension remained milky. It isimportant to note that after 105 min of attrition milling the averageparticle size in a good suspension was 20 nm and in a bad suspension was40 nm.

Example 1

This example provides a description of making and characterizingnanozirconia gels and zirconia green bodies of the present disclosure.

The suspension preparation, concentration and attrition-milling stepswere identical to Comparative Example 1. The said 55 wt % 2TODA zirconiasuspension were then further concentrated to form a gel using osmoticprocessing method as described below: 50-200 grams of 55 wt % (16.7 vol%) suspension was poured in a dialysis membrane tubing (Spectra/Pardialysis membrane MWCO 6000-8000, tubing flat width 50 mm), whileleaving ⅓ to ¼ of the tubing unfilled. The two ends of the tubing wereclosed by two clip closures. The loaded tubing was then immersed in a 20wt % polyethylene glycol (molecular weight 35,000) (PEG35K)/watersolution in a closed container. The container was then placed on ashaking table and continuously shaken at 125 rpm for 24 hours. Theobtained gel is translucent and soft, with 81 wt % (41.1 vol %) solidloading. The tactile feel of the gel is consistent with vacuum grease.The assessment of the gel softness and gel formability is presented inTable 3, with comparison to other examples.

Cylindrical PTFE molds of 27 to 37 mm in diameter and 30 to 40 mm inheight were prepared. Demolding agent was applied on the surfaces ofeach molds and then carefully wiped off, so that a thin layer of thedemolding agent was left on the surfaces. The said gel was furtherhomogenized with a Thinky Planetary Mixer at 1000 rpm for 60 seconds. 30to 60 gram zirconia gel was then fed into each mold with a Teflon coatedspatula. The loaded molds were put in a centrifuging machine (Legend XTby Thermo Scientific) and centrifugal cast at 4300 rpm for 5 minutes.The mold was then taken out and placed in an environmental chamber fordrying at 25° C. For the first 168 hours, the chamber humidity was setto be no lower than 80% and then gradually dropped to 20% in the next168 hours. The obtained dried zirconia green bodies were cylinders with24 to 32 mm in diameter and 14 to 18 mm in height, as shown in FIG. 10A.Green bodies that survived the said drying process had no visible cracksand bubbles. Survival rate of the green bodies 32 mm in diameter and 14to 18 mm in thickness is summarized in Table 3 in comparison with otherexamples. It should be noted while thickness of the samples made bydrop-casting was below 2 mm, thickness of the samples formed from gelwas exceeding 14 mm and approaching 20 mm.

TABLE 3 Examples of gels with PEG/PVA additives. Green Gel bodyloading*, Gel Gel PEG35K drying Processing Osmotic wt % softness/formability vs survival Suspension agent time, hr (vol %) rigidityranking PEG20K rate, % 2TODA EX1: 24 ~81 (41.1) soft 4 <70 No processingagent EX2: 24 ~76 (34.2) very soft 5 EX2A: 70-80 1.0% PEG 35k EX2B: >8020k EX3: 22 ~77 (35.4) soft 4 EX3A: >80 1.0% PEG + 35k 0.05% PVAEX3B: >80 20k EX4: 20 ~78 (36.8) soft 4 EX4A: <70 0.5% PEG + 35k 0.05%PVA EX4B: >80 20k EX5: 18 ~78 (36.8) semi-soft 2-3 EX5A: >80 0.1% PEG +35k 0.05% PVA EX5B: <70 20k EX6: 17 ~78 (36.8) semi-rigid 0 N.A. 0.05%PVA to rigid (too rigid) 2ESD EX7: 22 ~77 (35.4) semi-soft 2-3 0 Noprocessing agent EX8: 22 ~75 (33.0) soft to 3 EX8A: >80 1.0% PEGsemi-soft 35k EX8B: >80 20k EX9: 19 ~75 (33.0) soft to 3 EX9A: >80 1.0%PEG + semi-soft 35k 0.05% PVA EX9B: <70 20k EX10: 19 ~76 (34.2)semi-soft 2-3 EX10A: <70 0.5% PEG + 35k 0.05% PVA EX10B: <70 20k EX11:18 ~76 (34.2) soft to 3 EX11A: >80 0.1% PEG + semi-soft 35k 0.05% PVAEX11B: <70 20k EX12: 17 ~78 (36.8) Rigid 0 N.A. 0.05% PVA (too rigid)*Not the highest loading achievable. Higher gel loading can be achievedwith extended osmotic time.

Example 2

This example provides a description of making and characterizingnanozirconia gels and zirconia green bodies of the present disclosure.

The 2TODA zirconia suspension preparation, concentration,attrition-milling steps and gel preparation steps were identical toExample 1 with the same amount of osmotic time of 24 hours. However,after attrition milling and before placing the suspension to dialysistubing for osmosis, 1.0% by weight to solid zirconia of polyethyleneglycol with number average molecular weight of 35,000 (PEG35K) (Example2A) or 20,000 (PEG20K, obtained from SigmaAldrich) (Example 2B) wasadmixed to the suspension and homogenized on shaking table. The mixturewas then loaded in dialysis tubing and osmosized for 24 hours. Theobtained gel is translucent and very soft, with 76 wt % (34.2 vol %)solid loading. The gel was then centrifugal cast and dried identicallyas described in Example 1. The gel solid loading, gel softness, gelformability and green body survival rate are presented in Table 3.

The 24 mm diameter cylindrical green bodies obtained from example 2Awere ground to 3.5 mm thick and then subjected to organic burn out cyclein a burn-out furnace (Vulcan 3-550) by heating at a rate of 0.1° C./minto 240° C. and then 0.5° C./min to 700° C. and holding for 2 hours atthat temperature. The obtained “brown” bodies were then sintered in adental furnace or Nabertherm HTC 08/16 furnace at a ramp rate of 3°C./min to 1100° C., hold for 2 hours and then furnace cooled to idletemperature or to room temperature. After sintering, the samples are ˜18mm in diameter and ˜2.8 mm in thickness. The sintered samples werefurther ground and polished down to thickness of 1.0 mm to preparesamples for optical (transmittance and opalescence) measurements. Themeasurement results are presented in Table 2 and compared with those ofother examples. The number of sample size is 3 for optical measurements.

Example 3

This example provides a description of making and characterizingnanozirconia gels and zirconia green bodies of the present disclosure.

The 2TODA zirconia suspension preparation, concentration andattrition-milling steps and gel preparation steps were identical toExample 1 with osmotic time of 22 hours. However, after attritionmilling and before placing the suspension to dialysis tubing, 1.0% byweight to solid zirconia of PEG35K (Example 3A) or PEG20K (Example 3B)was admixed to the suspension, followed by admixing 0.05% by weight tosolid zirconia of polyvinylalcohol with weight average molecular weight9,000 to 10,000 (PVA, obtained from SigmaAldrich). After being shakenand homogenized, the mixture was then loaded in dialysis tubing andosmosized for 22 hours. The obtained gel was translucent and soft, with77 wt % (35.4 vol %) solid loading. The gel was then centrifugal castand dried identically as described in Example 1. The gel solid loading,gel softness, gel formability and green body survival rate are presentedin Table 3.

Example 4

This example provides a description of making and characterizingnanozirconia gels and zirconia green bodies of the present disclosure.

The 2TODA zirconia suspension preparation, concentration andattrition-milling steps and gel preparation steps were identical toExample 1 with osmotic time of 20 hours. However, after attritionmilling and before placing the suspension to dialysis tubing, 0.5% byweight to solid zirconia of PEG35K (Example 4A) or PEG20K (Example 4B)was admixed to the suspension, followed by admixing 0.05% by weight tosolid zirconia of PVA. After being shaken and homogenized, the mixturewas then loaded in dialysis tubing and osmosized for 20 hours. Theobtained gel was translucent and soft, with 78 wt % (36.8 vol %) solidloading. The gel was then centrifugal cast and dried identically asdescribed in Example 1. The gel solid loading, gel softness, gelformability and green body survival rate are presented in Table 3.

Example 5

This example provides a description of making and characterizingnanozirconia gels and zirconia green bodies of the present disclosure.

The 2TODA zirconia suspension preparation, concentration andattrition-milling steps and gel preparation steps were identical toExample 1 with osmotic time of 18 hours. However, after attritionmilling and before placing the suspension to dialysis tubing, 0.1% byweight to solid zirconia of PEG35K (Example 5A) or PEG20K (Example 5B)was admixed to the suspension, followed by admixing 0.05% by weight tosolid zirconia of PVA. After being shaken and homogenized, the mixturewas then loaded in dialysis tubing and osmosized for 18 hours. Theobtained gel was translucent and semi-soft, with 78 wt % (36.8 vol %)solid loading. The gel was then centrifugal cast and dried identicallyas described in Example 1. The gel solid loading, gel softness, gelformability and green body survival rate are presented in Table 3.

To measure the optical properties and biaxial flexural strength, twosizes of cylindrical green bodies of example 5A were prepared: 24 mmdiameter green bodies for optical measurements using 27 mm diameter PTFEmolds and 18 mm diameter green bodies for biaxial flexural strengthmeasurement using 22 mm diameter PTFE molds. Both cylindrical greenbodies were ground to 2.5 mm thick and then subjected to organic burnout cycle in a burn-out furnace (Vulcan 3-550) by heating at a rate of0.1° C./min to 240° C. and then 0.5° C./min to 700° C. and holding for 2hours at that temperature. The obtained “brown” bodies were thensintered in a dental furnace or Nabertherm HTC 08/16 furnace at a ramprate of 3° C./min to 1100° C., hold for 2 hours and then furnace cooledto idle temperature or to room temperature, respectively. Aftersintering, the samples were ˜18 mm and ˜13 mm in diameter, respectively,and ˜2.0 mm in thickness. The sintered samples were further ground andpolished down to thickness of 1.0 mm to prepare samples for optical(transmittance and opalescence) measurements and 1.0-1.2 mm for biaxialflexural strength measurements. The measurement results are presented inTable 2 and compared with those of other examples. The number of samplesize is 3 for optical measurements and at least 10 for biaxial flexuralstrength measurements.

Example 6

This example provides a description of making and characterizingnanozirconia gels and zirconia green bodies of the present disclosure.

The 2TODA zirconia suspension preparation, concentration andattrition-milling steps and gel preparation steps were identical toExample 1 with osmotic time of 17 hours. However, after attritionmilling and before placing the suspension to dialysis tubing, 0.05% byweight to solid zirconia of PVA was admixed to the suspension. Afterbeing shaken and homogenized, the mixture was then loaded in dialysistubing and osmosized for 17 hours. The obtained gel was translucent butrigid, with 78 wt % (36.8 vol %) solid loading. This rigid gel becamenon-formable, as described in Table 3.

Example 7

This example provides a description of making and characterizingnanozirconia gels and zirconia green bodies of the present disclosure.

2 kg of 55 wt % (16.7 vol %) aqueous suspensions of yttria (3 mol %)stabilized zirconia nanoparticulate, containing 0 to 0.3% alumina (e.g.,0.25% alumina) by weight to solid zirconia by weight to solid zirconiaand 2% of an ESD (2ESD suspension) by weight to solid zirconia. The pHof such a stabilized suspension was 5.1. The suspension was thenattrition milled with 50 μm diameter yttria-stabilized zirconia beads at3000 rpm rotation speed for 105 minutes, using Netzsch MiniCer attritionmilling machine. Dynamic Light Scattering (DLS) measurement (Nano-ZS byMalvern) was used to monitor the deagglomeration of particles during theattrition milling process, as shown in FIG. 6B.

The attrition milled 55 wt % 2ESD zirconia suspension were then furtherconcentrated to 75-80 wt % (33-40 vol %) zirconia content using theosmotic processing as described in Example 1. In this example, 22 hourosmotic time was applied and the obtained gel was translucent andsemi-soft, with 77 wt % (35.4 vol %) solid loading. The gel was thencentrifugal cast and dried identically as described in Example 1. Thegel solid loading, gel softness, gel formability and green body survivalrate are presented in Table 3.

Example 8

This example provides a description of making and characterizingnanozirconia gels and zirconia green bodies of the present disclosure.

The 2ESD zirconia suspension attrition milling and gel preparation stepswere identical to Example 7 with the same amount of osmotic time of 22hours. However, after attrition milling and before placing thesuspension to dialysis tubing, 1.0% by weight to solid zirconia ofPEG35K (Example 8A) or PEG20K (Example 8B) was admixed to the suspensionand homogenized on shaking table. The mixture was then loaded indialysis tubing and osmosized for 22 hours. The obtained gel wastranslucent and soft to semi-soft, with 75 wt % (33.0 vol %) solidloading. The gel was then centrifugal cast and dried identically asdescribed in Example 1. The gel solid loading, gel softness, gelformability and green body survival rate are presented in Table 3.

Example 9

This example provides a description of making and characterizingnanozirconia gels and zirconia green bodies of the present disclosure.

The 2ESD zirconia suspension attrition milling and gel preparation stepswere identical to Example 7 with osmotic time of 19 hours. However,after attrition milling and before placing the suspension to dialysistubing, 1.0% by weight to solid zirconia of PEG35K (Example 9A) orPEG20K (Example 9B) was admixed to the suspension, followed by admixing0.05% by weight to solid zirconia of PVA. After being shaken andhomogenized, the mixture was then loaded in dialysis tubing andosmosized for 19 hours. The obtained gel was translucent and soft tosemi-soft, with 75 wt % (33.0 vol %) solid loading. The gel was thencentrifugal cast and dried identically as described in Example 1. Thegel solid loading, gel softness, gel formability and green body survivalrate are presented in Table 3.

Example 10

This example provides a description of making and characterizingnanozirconia gels and zirconia green bodies of the present disclosure.

The 2ESD zirconia suspension attrition milling and gel preparation stepswere identical to Example 7 with osmotic time of 19 hours. However,after attrition milling and before placing the suspension to dialysistubing, 0.5% by weight to solid zirconia of PEG35K (Example 10A) orPEG20K (Example 10B) was admixed to the suspension, followed by admixing0.05% by weight to solid zirconia of PVA. After being shaken andhomogenized, the mixture was then loaded in dialysis tubing andosmosized for 19 hours. The obtained gel was translucent and semi-soft,with 76 wt % (34.2 vol %) solid loading. The gel was then centrifugalcast and dried identically as described in Example 1. The gel solidloading, gel softness, gel formability and green body survival rate arepresented in Table 3.

Example 11

This example provides a description of making and characterizingnanozirconia gels and zirconia green bodies of the present disclosure.

The 2ESD zirconia suspension attrition milling and gel preparation stepswere identical to Example 7 with osmotic time of 18 hours. However,after attrition milling and before placing the suspension to dialysistubing, 0.1% by weight to solid zirconia of PEG35K (Example 11A) orPEG20K (Example 11B) was admixed to the suspension, followed by admixing0.05% by weight to solid zirconia of PVA. After being shaken andhomogenized, the mixture was then loaded in dialysis tubing andosmosized for 19 hours. The obtained gel was translucent and soft tosemi-soft, with 76 wt % (34.2 vol %) solid loading. The gel was thencentrifugal cast and dried identically as described in Example 1. Thegel solid loading, gel softness, gel formability and green body survivalrate are presented in Table 3.

Example 12

This example provides a description of making and characterizingnanozirconia gels and zirconia green bodies of the present disclosure.

The 2ESD zirconia suspension attrition milling and gel preparation stepswere identical to Example 7 with osmotic time of 17 hours. However,after attrition milling and before placing the suspension to dialysistubing, 0.05% by weight to solid zirconia of PVA was admixed to thesuspension. After being shaken and homogenized, the mixture was thenloaded in dialysis tubing and osmosized for 17 hours. The obtained gelwas translucent but rigid, with 78 wt % (36.8 vol %) solid loading. Thisrigid gel became non-formable, as described in Table 3.

Example 13

This example provides a description of making and characterizingnanozirconia gels and zirconia green bodies of the present disclosure.

The 2TODA zirconia suspension preparation, concentration andattrition-milling steps and gel preparation steps were identical toExample 1 with osmotic time of 32 hours. However, after attritionmilling and before placing the suspension to dialysis tubing forosmosis, 1.0% by weight to solid zirconia of polyethylene oxide withaverage molecular weight of 100,000 (PEO100K, obtained fromSigmaAldrich) was admixed to the suspension and homogenized on shakingtable. The mixture was then loaded in dialysis tubing and osmosized for32 hours. The obtained gel was translucent and soft, with 74 wt % (31.8vol %) solid loading.

Cylindrical Silicone molds of 25 mm in diameter and 30 mm in height wereprepared. The said gel was further homogenized with a Thinky PlanetaryMixer at 1000 rpm for 60 seconds. About 30 gram gel was placed into eachmold with a Teflon coated spatula until it was totally filled. The moldwas then tightly capped with a silicone cup. The loaded molds were thensealed in plastic bags and placed in a Cold Isostatic Pressing (CIP)machine. The CIP pressure was set to 30 ksi for 5 minutes. The mold wasthen taken out and placed in an environmental chamber for drying asdescribed in Example 1. The gel solid loading, gel softness, gelformability and green body survival rate are presented in Table 4.

TABLE 4 Examples of tinders that didn't provide gels. Gel Gel Green bodyOsmotic Gel loading, softness/ formability drying survival SuspensionBinder time, hr wt % (vol %) rigidity ranking rate, % 2TODA EX13: 32 ~74(31.8) Soft 4-5 0 1.0% PEO100K EX14: 24 ~80 (39.6) Semi-soft 1 0 0.2% MCto semi- rigid EX15: 24 ~74 (31.8) soft to 3 0 1.0% semi-soft PEG35K +0.2% MC 2ESD EX16: 32 ~76 (34.2) Soft 4 0 1.0% PEO100K EX17: 22 ~76(34.2) semi-soft 2 0 0.2% MC EX18: 22 ~73 (30.7) soft to 3 0 1.0%semi-soft PEG35K + 0.2% MC Formation: CIP, 30 ksi, 5 mins

Example 14

This example provides a description of making and characterizingnanozirconia gels and zirconia green bodies of the present disclosure.

The 2TODA zirconia suspension preparation, concentration andattrition-milling steps and gel preparation steps were identical toExample 1 with osmotic time of 24 hours. However, after attritionmilling and before placing the suspension to dialysis tubing forosmosis, 0.2% by weight to solid zirconia of methylcellulose (MC,obtained from Dow Chemicals) with number average molecular weight of41,000 was admixed to the suspension and homogenized on shaking table.The mixture was then loaded in dialysis tubing and osmosized for 24hours. The obtained gel was translucent and semi-soft to semi-rigid,with 80 wt % (39.6 vol %) solid loading. The gel was then CIP cast anddried identically as described in Example 13. The gel solid loading, gelsoftness, gel formability and green body survival rate are presented inTable 4.

Example 15

This example provides a description of making and characterizingnanozirconia gels and zirconia green bodies of the present disclosure.

The 2TODA zirconia suspension preparation, concentration andattrition-milling steps and gel preparation steps were identical toExample 1 with osmotic time of 24 hours. However, after attritionmilling and before placing the suspension to dialysis tubing, 1.0% byweight to solid zirconia of PEG35K was admixed to the suspension,followed by admixing 0.2% by weight to solid zirconia of methylcellulose(MC). After being shaken and homogenized, the mixture was then loaded indialysis tubing and osmosized for 24 hours. The obtained gel wastranslucent and soft to semi-soft, with 74 wt % (31.8 vol %) solidloading. The gel was then CIP cast and dried identically as described inExample 13. The gel solid loading, gel softness, gel formability andgreen body survival rate are presented in Table 4.

Example 16

This example provides a description of making and characterizingnanozirconia gels and zirconia green bodies of the present disclosure.

The 2ESD zirconia suspension attrition milling and gel preparation stepswere identical to Example 7 with osmotic time of 32 hours. However,after attrition milling and before placing the suspension to dialysistubing for osmosis, 1.0% by weight to solid zirconia of PEO100K wasadmixed to the suspension and homogenized on shaking table. The mixturewas then loaded in dialysis tubing and osmosized for 32 hours. Theobtained gel was translucent and soft, with 76 wt % (34.2 vol %) solidloading. The gel was then CIP cast and dried identically as described inExample 13. The gel solid loading, gel softness, gel formability andgreen body survival rate are presented in Table 4.

Example 17

This example provides a description of making and characterizingnanozirconia gels and zirconia green bodies of the present disclosure.

The 2ESD zirconia suspension attrition milling and gel preparation stepswere identical to Example 7 with osmotic time of 22 hours. However,after attrition milling and before placing the suspension to dialysistubing for osmosis, 0.2% by weight to solid zirconia of methylcellulose(MC) was admixed to the suspension and homogenized on shaking table. Themixture was then loaded in dialysis tubing and osmosized for 22 hours.The obtained gel was translucent and semi-soft, with 76 wt % (34.2 vol%) solid loading. The gel was then CIP cast and dried identically asdescribed in Example 13. The gel solid loading, gel softness, gelformability and green body survival rate are presented in Table 4.

Example 18

This example provides a description of making and characterizingnanozirconia gels and zirconia green bodies of the present disclosure.

The 2ESD zirconia suspension attrition milling and gel preparation stepswere identical to Example 7 with osmotic time of 22 hours. However,after attrition milling and before placing the suspension to dialysistubing, 1.0% by weight to solid zirconia of PEG35K was admixed to thesuspension, followed by admixing 0.2% by weight to solid zirconia ofmethylcellulose (MC). After being shaken and homogenized, the mixturewas then loaded in dialysis tubing and osmosized for 22 hours. Theobtained gel was translucent and soft to semi-soft, with 73 wt % (30.7vol %) solid loading. The gel was then CIP cast and dried identically asdescribed in Example 13. The gel solid loading, gel softness, gelformability and green body survival rate are presented in Table 4.

Example 19

This example provides a description of making and characterizingnanozirconia gels of the present disclosure.

The 2ESD zirconia suspension attrition milling and gel preparation stepswere identical to Example 7 with osmotic time of 16 hours and smallerMWCO dialysis membrane tubing (MWCO 1000). However, after attritionmilling and before placing the suspension to dialysis tubing, 0.05% byweight to solid zirconia of polyacrylic acid with weight averagemolecular weight 1800 (PAA, obtained from SigmaAldrich) was admixed tothe suspension. After being shaken and homogenized, the mixture was thenloaded in dialysis tubing and osmosized for 16 hours. The obtained gelwas translucent and semi-soft, with 77 wt % (35.4 vol %) solid loading.

Example 20

This example provides a description of making and characterizingnanozirconia gels of the present disclosure.

The 2ESD zirconia suspension attrition milling and gel preparation stepswere identical to Example 7 with osmotic time of 16 hours and smallerMWCO dialysis membrane tubing (MWCO 1000). However, after attritionmilling and before placing the suspension to dialysis tubing, 1.0% byweight to solid zirconia of PEG35K was admixed to the suspension,followed by 0.05% by weight to solid zirconia of polyacrylic acid (PAA).After being shaken and homogenized, the mixture was then loaded indialysis tubing and osmosized for 16 hours. The obtained gel wastranslucent and soft to semi-soft, with 75 wt % (33.0 vol %) solidloading.

Example 21

This example provides a description of making and characterizingnanozirconia gels, intermediate materials, and dental restorations ofthe present disclosure.

Gel prepared from Example 5A was centrifugal cast and dried as describein Example 1. The obtained green bodies were cylinders with ˜32 mm indiameter and 14-18 mm in height and free of cracks or bubbles by visualexamination. One of the sides of these cylindrically shaped blanks wasflattened and glued to Katana-compatible mandrel by superglue. KatanaDental CAD/CAM System (Model H18) was used to mill these green blanksinto standard molar crowns using enlargement factors of 1.15-1.2. Theobtained crack-free green crowns were then subjected to organicsburn-out by heating at a rate of 0.1° C./min to 240° C., then 0.5°C./min to 700° C. and holding for 2 hours at that temperature resultingin a crack free “brown” crowns. The “brown” crowns were then sintered ata ramp rate of 3° C./min to 1100° C., hold for 2 hours. The as-sinteredmolar crowns (Example 21A) were then glazed (Example 21B) or stained andglazed to match Vita A1 shade guide as shown in FIG. 10B,C (Example21C). The sample information is summarized in Table 5.

TABLE 5 Dental restorations milled from gel-formed green bodies andfinished using conventional equipment and procedures. Disper- ProcessingExample sant agent Sintering Post treatment EX21: 2TODA 0.1% 1100° C.EX21A: As sintered crowns pro- PEG35K + for 2 hrs, EX21B: Glazed ducedwith 0.05% 3° C./ EX21C: Stained to Vita gel formed PVA min A1 shade andglazed by EX5A EX22: 2TODA 1.0% 1100° C. EX22A: Glazed crowns pro-PEG35K + for 2 hrs, EX22B: Stained to Vita duced with 0.05% 3° C./ A3shade and glazed gel formed PVA min by EX3A

Example 22

This example provides a description of making and characterizingnanozirconia gels, intermediate materials, and dental restorations ofthe present disclosure.

Gel prepared from Example 3A was centrifugal cast and dried as describein Example 1. The obtained green bodies were cylinders with ˜32 mm indiameter and 14-18 mm in height and free of cracks or bubbles by visualexamination. One of the sides of these cylindrically shaped blanks wasflattened and glued to Katana-compatible mandrel by superglue. KatanaDental CAD/CAM System (Model H18) was used to mill these green blanksinto standard molar crowns using enlargement factor of 1.15-1.2. Theobtained crack-free green crowns were then subjected to organicsburn-out by heating at a rate of 0.1° C./min to 240° C., then 0.5°C./min to 700° C. and holding for 2 hours at that temperature resultingin a crack free “brown” crowns. The “brown” crowns were then sintered ata ramp rate of 3° C./min to 1100° C., hold for 2 hours. The as-sinteredmolar crowns were then glazed (Example 22A) or stained and glazed tomatch Vita A3 shade guide. The sample information is summarized in Table5.

Example 23

This example provides a description of making and characterizingnanozirconia gels and zirconia green bodies of the present disclosure.

This example is identical to Example 7 except for a longer osmotic time(24 hours vs. 22 hours) when gel was prepared. The longer osmotic timehas increased the gel loading from 77 wt % (Example 7) to 80 wt %(Example 23), which resulted in the gel turning from semi-soft (Example7) to semi-rigid/rigid (Example 23). The obtained gel was thusnon-formable. This example demonstrated that the gel formability ishighly related to gel loading, which can be controlled by osmotic time.The gel solid loading, gel softness and gel formability are presentedand compared in Table 6.

TABLE 6 Comparison of 2ESD gels with different gel solid loading GreenGel body loading, Gel Gel PEG35K drying Processing Osmotic wt %softness/ formability vs survival Suspension agent time, hr (vol %)rigidity ranking PEG20K rate, % 2ESD EX23: 24 ~80 (39.6) semi-rigid 0N/A with higher No to rigid solid processing loading agent EX24: 24 ~76(34.2) soft to 3 EX24A: 0 1.0% PEG semi-soft 35k EX24B: 0 20k EX25: 22~77 (35.4) soft to 3 EX25A: 0 1.0% PEG + semi-sof 35k 0.05% PVA EX25B: 020k EX26: 20 ~78 (36.8) semi-soft 2 EX26A: 0 0.5% PEG + 35k 0.05% PVAEX26B: 0 20k EX27: 18 ~78 (36.8) semi-soft 0-1 EX27A: 0 0.1% PEG + to35k 0.05% PVA semi- EX27B: 0 rigid 20k 2ESD EX7: 22 ~77 (35.4) semi-soft2-3 0 with No lower processing solid agent loading EX8: 22 ~75 (33.0)soft to 3 EX8A: >80 1.0% PEG semi-soft EX8B: >80 EX9: 19 ~75 (33.0) softto 3 EX9A: >80 1.0% PEG + semi-soft 35k 0.05% PVA EX9B: <70 20k EX10: 19~76 (34.2) semi-soft 2-3 EX10A: <70 0.5% PEG + 35k 0.05% PVA EX10B: <7020k EX11: 18 ~76 (34.2) semi-soft 2 EX11A: >80 0.1% PEG + 35k 0.05% PVAEX11B: <70 20k

Example 24

This example provides a description of making and characterizingnanozirconia gels and zirconia green bodies of the present disclosure.

This example is identical to Example 8 except for a longer osmotic time(24 hours vs. 22 hours) when gel was prepared. The longer osmotic timehas increased the gel loading from 75 wt % (Example 8) to 76 wt %(Example 24). Both gels were in the range of soft to semi-soft but thegel in example 8 was a little softer due to the lower solid loading. Thegel solid loading, gel softness, gel formability and green body survivalrate are presented in and compared Table 6.

Example 25

This example provides a description of making and characterizingnanozirconia gels and zirconia green bodies of the present disclosure.

This example is identical to Example 9 except for a longer osmotic time(22 hours vs. 19 hours) when gel was prepared. The longer osmotic timehas increased the gel loading from 75 wt % (Example 9) to 77 wt %(Example 25). Both gels were in the range of soft to semi-soft but thegel in example 9 was softer due to the lower solid loading. The gelsolid loading, gel softness, gel formability and green body survivalrate are presented in and compared Table 6.

Example 26

This example provides a description of making and characterizingnanozirconia gels and zirconia green bodies of the present disclosure.

This example is identical to Example 10 except for a longer osmotic time(20 hours vs. 19 hours) when gel was prepared. The longer osmotic timehas increased the gel loading from 76 wt % (Example 10) to 78 wt %(Example 25). Both gels were in the range of semi-soft but the gel inexample 10 was softer due to the lower solid loading. The gel solidloading, gel softness, gel formability and green body survival rate arepresented in and compared Table 6.

Example 27

This example provides a description of making and characterizingnanozirconia gels and zirconia green bodies of the present disclosure.

This example is identical to Example 11 with the same osmotic time (18hours) when gel was prepared. However, the gel prepared from thisexample is 2 wt % higher in solid loading than that from example 11 (78wt % vs. 76 wt %). The reason is not clear at this time but it could bedue to the relatively lower suspension to osmotic solution volume ratiospecifically in Example 27. The 2 wt % higher solid loading resulted inthe gel turned from semi-soft (example 11) to semi-rigid (Example 27)and led to different results of survival rate as indicated in Table 6.

Example 28

This example provides a description of making and characterizingnanozirconia gels of the present disclosure.

Gels prepared from Example 1, 2A, 5A (2TODA suspension) and Example 23,8A, 11A (2ESD suspension) were characterized with a Kinexus Ultra+Rotational Rheometer by Malvern, to measure their yield stresses andshear rate dependent viscosities. All tests were done at 25° C. 25 mmroughened parallel plates were employed to provide improved adhesion tohold the gel samples. A liquid seal enclosure was used to provide ahumid environment to avoid gel drying during the tests. The thickness ofthe gel samples (working gap between plates) were controlled by a fixednormal load of 5 N. The rheometry measurement was conducted by graduallyincreasing the shear stress from 5 Pa to a maximum of 10,000 Pa at arate of 10 Pa per second. The testing results are presented in FIGS. 7A,7B, 8A, and 8B, and the yield stress and viscosity at yield point ofeach gel are summarized in Table 1A.

Example 29

This example provides a description of making and characterizingnanozirconia gels and zirconia green bodies of the present disclosure.

Vickers hardness (HV) measurements were conducted on dried green bodiesformed with different forming methods, including drop castingnanozirconia suspensions (comparative example 1), centrifugal castingnanozirconia gels (Example 1 an Example 5A) and CIP-ing conventionalzirconia powders (Zirlux® FC2 Green discs). A Mitutoyo HV-112indentation hardness testing machine was used for the measurements.Sample hardness was measured at different load levels of 1.0, 2.5, 5.0,and 10.0 kg force. All samples were 7-8 mm in thickness and at least 10times thicker than the indentation diagonals with at least 5 indentationdiagonals of spacing between indents. Five consecutive measurements wereconducted on each sample and the results are summarized in Table 7.

TABLE 7 Comparison of Vickers hardness of different green bodies.Vickers Hardness (HV), kg/mm² Average ± Standard Deviation for fiveconsecutive measurements. All samples were 7-8 mm in thickness (samplethickness is given in brackets below) and at least 10 times thicker thanthe indentation diagonal with at least 5 indentation diagonals ofspacing between indents. Centrifugally cast Zirlux FC2 disc usingformable CIP-ed at 380 MPa nanozirconia gel (commercial Drop cast 0.1%product tested in No No PEG35K + green stage prior processing processing0.05% Load, to soft sintering to agent agent PVA*** kg HV of 50-70kg/mm²) (7.7 mm) (7.7 mm) (7.4 mm) 1.0 24.3 ± 0.2 19.0 ± 0.8 42.5 ± 0.643.2 ± 0.2 2.5 24.3 ± 0.2 18.9 ± 0.5 46.3 ± 0.5 45.0 ± 0.2 5.0 —  20.0 ±0.5/ 44.3 ± 0.3 46.5 ± 0.3 19.5** 10.0* 24.5 ± 0.1 Samples 44.8 ± 0.646.4 ± 0.2 cracked into multiple pieces *Surviving test at 10 kg load isnecessary condition for machinability as peak milling loads are higher**Average for two measurements on one of the samples which cracked evenat 5 kg load during the third consecutive measurement ***Confirmedmillable

Example 30

This example provides a description of making and characterizing “brown”bodies of the present disclosure.

While typically brown blanks are translucent as shown in FIG. 3 they canbe made black or brown and totally impermeable to light indicatingincomplete removal of organics. That where the term “brown body”originally came from. Most of the times these samples are irreversiblylost due to ash/soot formation within the pores. Noteworthy that in somecases brown color shown in FIG. 14 can be reversible and still produceredeemable brown bodies. It is speculated that the brown color of thissample was resulted from the oxidation of organics (TODA processingagent) in the green body.

Example 31

This example provides a description of making and characterizingnanozirconia gels of the present disclosure.

Gels prepared from Examples 1, 4A and 5A were characterized with AgilentCary 100 UV-Visible Spectrophotometer G9821A to measure thetransmittance, and Minolta CM3610D Spectrophotometer to measure theopalescence. The gels were placed between two 2″ diameter quartz discswhile 1.4 mm thick spacers was used to control the gel thickness. Themeasurement results are summarized in Table 8.

TABLE 8 Optical properties of examples of gels. Total forward In-lineExamples 2TODA, Transmittance Transmittance Opal- (Thickness processing(%) @560 nm (%) @560 nm escence is 1.4 mm) agent wavelength wavelengthparameter EX1  No 72.4 58.9 26.4 processing agent EX4A 0.05 PVA + 76.552.8 28.0 0.5 PEG35k EX5A 0.05 PVA + 75.8 61.8 28.4 0.1 PEG35k

Example 32

This example provides a description of making and characterizingzirconia green bodies of the present disclosure.

Green body cylinders prepared from Example 4A, 5A and 4B were cut,ground, and polished to make 2.0 mm thick discs-shaped samples fortransmittance and opalescence measurements using the same equipment ofExample 31. The measurement results are summarized in Table 9.

TABLE 9 Optical properties of examples of green bodies. Examples 2TODA,Total forward (Thickness processing Transmittance (%) Opalescence is 2.0mm) agent @560 nm wavelength parameter EX4A 0.05 PVA + 46.9 20.9 0.5PEG35k EX5A 0.05 PVA + 47.8 21.1 0.1 PEG35k EX4B 0.05 PVA + 48.4 19.90.5 PEG20k

Example 33

This example provides a description of making and characterizingzirconia brown bodies of the present disclosure.

Disc-shaped brown/pre-sintered bodies were prepared by heating greenbody discs of Example 31 to 550° C. and cooling down to roomtemperature, for transmittance and opalescence measurements using thesame equipment of Example 31. The measurement results are summarized inTable 10.

TABLE 10 Optical properties of examples of brown bodies. Example 2TODA,Total forward Opal- (Thickness processing Transmittance (%) escence is2.0 mm) agent @560 nm wavelength parameter EX4A 0.05 PVA + 0.5 PEG35k43.4 29.0 EX5A 0.05 PVA + 0.1 PEG35k 44.3 28.3 EX4B 0.05 PVA + 0.5PEG20k 43.9 28.5

Example 34

This example provides a description of making and characterizingnanozirconia gels of the present disclosure.

Gels prepared from Example 1, 4A and 5A were diluted with water to 0.5wt % loading. Particle size and size distribution were measured withMalvern Nano-ZS zeta-sizer. The measurement results are compared withthose of the original suspension used to make the gel, as shown in Table11.

TABLE 11 Example of redispersibility of gels. Volume Average D, nmProcessing agent(s) Suspension Gel No processing agent 22.9 ± 1.4 23.5 ±4.3 (EX1) 0.5% PEG35K + 22.8 ± 1.5 25.3 ± 3.3 0.05% PVA (EX4A) 0.1%PEG35K + 23.2 ± 1.3 22.1 ± 4.1 0.05% PVA (EX5A)

The invention claimed is:
 1. A method of making a gel comprising aplurality of zirconia nanoparticles and water, wherein the zirconiananoparticles have an average size of 10 to 30 nm, 95% or more of thezirconia nanoparticles by volume have a size of 45 nm or less, thezirconia nanoparticles are present at 70 to 85% by weight based on thetotal weight of the gel, and the gel is a formable gel, wherein the gelexhibits a viscosity at yield point of 1×10⁹ to 12×10⁹ mPa·s and a yieldstress of 1×10³ to 9×10³ Pa, the method comprising: a) providing anaqueous suspension comprising zirconia nanoparticles having an averagesize of 10 to 30 nm and 95% or more of the zirconia nanoparticles byvolume have a size of 45 nm or less, wherein the zirconia nanoparticlesare present at less than 70% by weight of the aqueous suspension; and b)concentrating the aqueous suspension by removing water from the aqueoussuspension with a semipermeable membrane, wherein the water removal isdriven by intrinsically induced pressure or externally imposed pressure,until the suspension has zirconia nanoparticles present at 70 to 85% byweight of the aqueous suspension, whereby the gel is formed.
 2. Themethod of claim 1, wherein the aqueous suspension further comprises aprocessing agent.
 3. The method of claim 1, further comprising:attrition milling a starting aqueous suspension comprising zirconiananoparticles having an average size greater than 50 nm, wherein thezirconia nanoparticles are present at 50% or greater by weight of thestarting aqueous suspension based on the total weight of the startingaqueous suspension, and, optionally, subjecting the attrition milledstarting aqueous suspension to centrifugation, to provide the aqueoussuspension of a).
 4. The method of claim 3, wherein the starting aqueoussuspension prior to attrition milling further comprises a colloidalstabilizer and/or a particle interaction strengthening agent is added tothe starting aqueous suspension after attrition milling.
 5. The methodof claim 1, wherein a starting aqueous suspension comprises zirconiananoparticles having an average size greater than 50 nm, wherein thezirconia nanoparticles are present at less than 50% by weight of thestarting aqueous suspension based on the total weight of the startingaqueous suspension and the method further comprises: i) concentratingthe starting aqueous suspension by heating and/or applying sub-ambientpressure to the starting aqueous solution until the zirconiananoparticles are present at 50% or greater by weight based on the totalweight of the starting aqueous suspension; and ii) attrition milling theconcentrated starting aqueous suspension from i) and, optionally,subjecting the attrition milled starting aqueous suspension tocentrifugation, to provide the aqueous suspension of a).
 6. The methodof claim 5, wherein a colloidal stabilizer is added to the startingaqueous suspension prior to the concentration of the starting aqueoussuspension and/or a particle interaction strengthening agent is added tothe starting aqueous suspension after attrition milling.
 7. The methodof claim 1, wherein the concentrating is carried out by an osmoticprocess comprising an osmotic solution.
 8. The method of claim 7,wherein the osmotic solution is an aqueous polymer solution.
 9. Themethod of claim 7, wherein the osmotic process is carried out withphysical agitation of the aqueous suspension.
 10. The method of claim 7,wherein the osmotic process comprises: a) placing the aqueous suspensioncomprising zirconia nanoparticles in an enclosure at least a portion ofan external surface of which is a semipermeable membrane; b) contactingthe enclosure with an osmotic solution such that the osmotic solutionand the aqueous suspension are in fluid contact; c) agitating theaqueous suspension; and d) optionally, heating the osmotic solution. 11.The method of claim 1, wherein the concentrating is carried out by atangential flow filtration process.
 12. The method of claim 11 whereinthe tangential flow filtration process comprises: a) flowing the aqueoussuspension comprising zirconia nanoparticles through a channel at leasta portion an external surface of which is the semipermeable membrane; b)repeating a) until the a desired amount of water is removed from theaqueous suspension comprising zirconia nanoparticles; and c) optionally,heating the aqueous suspension comprising zirconia nanoparticles. 13.The method of claim 12, wherein the semipermeable membrane is contactedwith an osmotic solution.
 14. The method of claim 1, wherein: i) 99% ofnanoparticles by volume have a size less than 60 nm±10 nm; ii) 95% ofnanoparticles by volume have a size less than 40 nm±5 nm; iii) 50% ofnanoparticles by volume have a size less than 20 nm±5 nm; and iv.) 5% ofnanoparticles by volume have a size less than 12 nm±3 nm.